I'm wondering how hard this would be to implement, or if an implementation exists. Ideally for me it would be 3 separate sites, and when a registration occurs the information is sent to all three sites, and afterwards, obviously, the user would be able to log in with their details on any of the sites. And perhaps during registration have a Yes/No radio box whether or not to do this, where Yes would run the hopper script and No would proceed with normal registration. Just a thought, I'm no programmer, though if someone can do it they're free to run with the idea. I was thinking maybe have one SQL database, held in common by all sites in the hopper, that holds only user info, while the rest of the site is governed as usual. This is just me speculating, I don't know how to do it or if it can be done.

This concept is covered in two separate ways in this XOOPS Wiki post. (One is your way with the single database, the other is a bit more complicated, I think.) Keep in mind that a single database could create problems if you want to keep your content entirely separate.

I hope the Wiki file helps! :)