Is it possible to merge users between multiple sites in a site network?

[PanamaRed]

I have a funky idea to merge users of multiple sites into a site network. Idea is to create cross traffic between multiple sites that are based around the same thing but different aspects of that same thing.

I don't want the sites to all be one, or merge the data bases I want to basically make navigating the site into navigating many sites in a sense.. A portal that wraps say 4 or 5 sites with merged members and a unified look.

For example I have a reptile site that without trying to be focused on one species of reptile is for the most part.. my site has loads of boa constrictor traffic As it sits I know several others with similar VB based sites in my same their focus is on other species, so we really don't compete.. So instead of having to navigate to a different site via your book marks history or what ever, and be logged in as a different user, you just click the link to the other species section and are still logged in and on the other site with it's focus on that species..

This would help ad sales as well as form massive cross traffic, and I don't have to deal with that much more BS from the increased traffic and moderation...

Is any of this possible?

Thanks in advance.