It shouldn't be possible. Top anchor ads add a padding, equal to the height of the ad, to the top of the page, pushing all the content down. As a result there shouldn't be any content, ads or otherwise, under the ad at page load. But, obviously as the page is scrolled content will appear below the anchor ad, including other ads and that is fine.

Technical aside, why not go for the best user experience? Things that overlap tend to drive visitors off!