If the data is the same on each is for an umbrella company/site, and the domain name will help drive traffic to the combination, that seems to be what redirections are for ... to a THIRD umbrella site (the one you want to promote).

Seems like a lot of effort for what will essentially be a single domain BRAND ...

Real world? Two domain names=two sites, each different as far as serps are concerned. (Not mentioning the two ips involved ... they ARE separate, correct?)

The UI, structure, and overall concept will be the same, the only real difference will be the target niche, data.

this is equivalent to saying the two sites are the same type of forum - the only real difference is one is about gardening and the other is about fly fishing.

What is the best way to integrate these two sites? Is this bad idea?

if by "integrate" you mean "link" you should do it in a way that is useful for the visitor.

If the data is the same on each is for an umbrella company/site

The data is not the same, the data is different. Its like baseball and hockey, or to take phranque's example fly fishing vs deep sea fishing. To some extent there is cross over but the topics are distinct. The format of one site is applied to the topic of the second. There will be a subset of the target audience that is interested in both topics, but most users will be focused on their specific topic of interest.

that seems to be what redirections are for ... to a THIRD umbrella site (the one you want to promote).

Redirection seems like a solution, but as you (@tangor) point out, then the whole endeavor is pointless. The domain name is a play on words and for it to work and make any sense the topic, as described by the gTLD needs to appear after the domain name. If I redirect then that whole construct is lost. Most users don't come to the site via the home page but instead land on specific pages, something like:
fancy-website-about.widget/model-xyz-widgets or
fancy-website-about.widget/model-abc-widgets
so redirecting gadget to the widget domain will be nonsensical:
fancy-website-about.widget/model-abc-gadgets

and redirect to a "neutral" domain:
fancy-website-about.com/model-abc-widgets
fancy-website-about.com/model-abc-gadgets
is also nonsensical because without the play on words the domain name has no relevance to either niche, that and the .com or other common TLDs are either taken or way out of my price range.

(Not mentioning the two ips involved ... they ARE separate, correct?)

No they are not separate, they would both be run of the same box and sharing the same backend and IP.

if by "integrate" you mean "link" you should do it in a way that is useful for the visitor.

Yes I mean link, and if possible make it clear to Google that these sites are related if not the same. I could easily link the sites, with a link in the nav or footer and then other place as is relevant.

Yes I mean link, and if possible make it clear to Google that these sites are related if not the same. I could easily link the sites, with a link in the nav or footer and then other place as is relevant.

exactly - there should be a way to link them in a way that is relevant and useful.
disney cruises is likely to link to disneyland in a global disney corporate nav in the header and/or footer.
the cubs will likely link to the dodgers in mlb.com game schedules and results.
dictionary.com will link to thesaurus.com for the same word.
brand.music should link to brand.photos in a way that makes sense for "brand" and is relevant for "brand" visitors.

I bought the second domain name today. So now I trying to figure out how I can integrate both websites. Essentially the websites will be identical, run off the same server and backend. The UI, structure, and overall concept will be the same, the only real difference will be the target niche, data.

What is the best way to integrate these two sites? Is this bad idea?

You probably have ideas of linking both sites to each other to quickly rank the second site and help the first site. It will sit on the same server, have the same IP. The ownership of the domains will be the same. This is the beginnings for a PBN. It will be there just for manipulating the rankings.

Google will penalize you for this.

This is a very bad idea.

To have any chance of this working you need to use a different server so you have a different IP, hosting is cheap. You need to use private domain registration to hide ownership. Fail to do either of these two things will be your downfall.

Edited add.
Never cross domain link sites in the main navigation. It is deceptive to the USER because they click a link in the main nav and end up on a different site. This is know as a deliberate attempt to manipulate the rankings and will get you penalized "FAST".

To have any chance of this working you need to use a different server so you have a different IP, hosting is cheap. You need to use private domain registration to hide ownership. Fail to do either of these two things will be your downfall.

And use different Adsense, GA, GSC, etc... accounts, and do not access these accounts from the same IP, otherwise Google will easily be able to figure out that the two sites have the same owner or at least related... but in all events, if the two sites are very close, Google will guess it anyhow. I don't think that different IP addresses and hidden registration information can fool Google nowadays... and if Google finds out that you are making so much efforts to hide the relation between the two sites, it might finds this suspect ... Keep in mind that Google knows everything about everybody.

Personally, I have one big site, and a hobby one, hosted on the same IP , there are "some" interlinking , and I never had any problems... I think that the point is that, the interlinking has to be legitimate and natural. If you interlink things which are unrelated, or are the exact same, this is not a good idea. Does a link brings something interesting to the visitor , or not, that is the question to ask.

@KnowOneSpecial

You probably have ideas of linking both sites to each other to quickly rank the second site and help the first site.

This is not the case. The current site ranks poorly as it is, so there is no ranking benefit to be had. As I explained the point is to expand the reach into a new niche, that is it, no SEO mumbo jumbo. The goal is not to obfuscate ownership, but to the contrary, have both sites branded as one, with one site for each niche.

@NickMNS

Maybe I misunderstood this... you tell me

Yes I mean link, and if possible make it clear to Google that these sites are related if not the same. I could easily link the sites, with a link in the nav or footer and then other place as is relevant.

Cross domain linking in the main navigation of a site can only be construed as rank manipulation in my opinion, but you do as you want.

@KnowOneSpecial ...

Welcome to Webmasterworld! Nice to see new faces. :)

@NickMNS ...

I get what you are attempting ... but having only one IP between the two it will be near impossible to "grow" either.

I have done something SIMILAR on a single domain for a topic that has multiple niches by creating a NICHE page that then has it's own "niche" content ...It was so successful in driving traffic directly to that "niche" that I added a few more!

/example.com/nichetopic
/example.com/nichetopic2
/example.com/nichetopic3

All under the same IP and domain ... but the difference was MENTALLY I coded and treated each of those niche groups AS IF they were separate domains in every way EXCEPT for being an actual domain.

That said, you can register your play on words domain to prevent anyone from using it ... and never actually make a domain UNTIL YOU HAVE ESTABLISHED TRAFFIC for it in the first place.

Just some thoughts ... but as expressed so far, this looks like "cake and eat it too" and that rarely works with any se, much less g.

I may be wrong here, but would it not solve the problem using canonical URLs?

Basically choose a "primary" domain. Using whatever mechanism for targeting audiences you want, but any content being displayed on a domain that is not the primary then you ensure that you include a canonical entry to the primary domain. Search for the

rel=”canonical” link element

It was a standard created in 2009 and is meant to be used to allow the same content to be published across multiple domains without duplicate content penalties.
[support.google.com...]

The one trap to be careful of using this is that it can affect which domain comes up in organic searches, but it is all outlined in the documentation.

I don't see the issue with a single IP.

The thing is that I'm using a gTLD so the current site is example.niche1 so following the subfolder structure isn't ideal. The result would be example.niche1/niche2. Thus to make it work I would have to move the site to new .com domain, redirect the old domain and then build it using the folder structure. But I have my doubts about that approach.

@DeMaestro the two sites have completely different content, there is nothing canonical about them. I want both sites to rank for their respective niches.

I wasn't talking about subdomains ... as those are ALWAYS linked to the domain

I spoke of creating the NICHE INSIDE the existing domain, finding the audience, THEN, when it made sense, DO the separate host with the new play on words domain and REDIRECT the existing to the new. This way you test the waters FIRST to see if there is enough traffic/etc to justify the expense, or know it will at least pay for itself.

This is no different than say a book publishing site that has dedicated "niches" for either genres, or specific authors who then decide to break those out to domains of their own ...

At any rate, you can always try it and see how it flies. Report back on how that worked out!