Bridge between two Dolphin sites

I am looking for a bridge between two Dolphin sites. I have two sites, but users would like to register only one site.

How can I do this?

Thanks in advance!

I believe this was discussed before, so you may want to search the forums.

I foun this: http://www.boonex.com/forums/topic/Two-Dolphin-Sites-Share-Only-Username-Database-POSSIBLE-.htm

But his module is a bit expensive: http://www.boonex.com/m/Cross_Database_Profile_Synchronization

Cheaper mode?