I'd think that geo-separation of hosting and of inbound link sources to both sites would be a big factor. You'd definitely want a bunch of .ca inbounds to the .ca site... and I would not interlink the two sites very much.

If you wanted to direct the US user from, say, each page of the .ca site to the .com site, or vice versa, I'd recommend javascript links.

Google is smart enough to sort this out and be visible in the proper respective countries.

I've been present when some of those assurances were offered, and read same also. Unfortunately I have not seen really good solutions to the issue, and certainly 'trusting' the SE algo's to get it right with largely undifferentiated content is NOT one of the best choices in this case. Plus while G has at times indicated as RC says, at other times they've acklowledged issues in this area.

One of the common issues that comes up with the international sites I've been working with lately relates to this question, and the reality is, there are lots of problems with how this issue is getting sorted on the SE site of things. One version ranking and the other not, and duplicate content affecting both sites, are common complaints.

There is no silver bullet in this areas AFAIK, but there are some options that can increase chances of ranking well:

1) Stick with domains that are country specific. Those .com's were interpreted by the U.S. community as being somewhat U.S.-centric, but that is not true. They are international in nature and should be treated as such.

2) The best way to avoid issues -- if your choice is a .com plus one or more country specific domains -- is to offer sites in different languages. One current client of mine has a Spanish language site with a local domain, and a .com in English language. They link conservatively between the two, or hide links on subpages where flipping between language pages makes sense, and have no problems

3) Sites that contain few differences in language and page contents between local country domains and .com's are, like it or not, subject to potential issues. I'll be thrilled when that is no longer the case, but so far I don't see reliable SE interpretation in this area.

4) Certainly, getting local Web hosts helps mitigate the chances of issues. Put the .ca site on a Canadian host. Put the .com in the U.S. or in this case perhaps the UK.

5) If you can add lead-ins to the pages that are local or country specific that helps too, as does the use of local country address on the bottom of each page of the site

6) Any other things you can do to differentiate are also helpful. Language differences; promotional items that can appear sitewide, etc.

Language differences...

I once was filming with a BBC producer who chided his on-camera talent, a Brit who'd lived in the US for a while: "Bernard, that's not English. That's not even American!" Unfortunately, on the web, Amurrican, Canadian, British, and Australian English look a lot alike. As you point out, that's problems. It's also hard to get US clients to drop their dot-com in favor of a dot-us for local consumption.

I think what I'd emphasize beyond what you've suggested is, if possible, to promote the country-specific TLDs for the English language sites within their local countries only.

If you start out the other sites with href links from the dot-com domain, I think the issue is already clouded for Google. Much as I hate javascript for navigation, that's the way I'd link among the English language domains.

The problem, as outlined by the OP: Almost no uniuqe contect between two English lauguage sites.

Personally, would not at all give up on the .com. Rather, I'd go out of my way to design the pages between the two sites to differentiate them as much as possible. I would also take advantage of technical factors like hosting the sites in different countries, and most certainly try to get mainly Canadian sites links to the .ca domain, and non-Canadian sites to link to the .com.

Point being, be sure to provide ample signals to the SE's as to how the country specific sites/pags and the counterpart .com pages are different. People aren't doing that enough, and this thing about trusting the SE's to sort out sites with little differentiation other than the TLD is risky at best.