I am looking for a way for my users to be able to login to both of my Joomla sites (one running 1.5 and the other 1.6) using the same credentials. For example, if a user registers to my Joomla1.5 portal, when they use that same info for my Joomla1.6, they get logged in without having to re-register and vice versa.
I guess an authentication plugin would be the best solution in combination with a XML-RPC service? Joomla1.5 site's plugin is triggered which "communicates" with the Joomla1.6 site where the user is already registered, if the credentials check out, a new user is created and the user automatically gets logged in.
Thanks in advance for any ideas/help.
You are trying to do two very different things that will require two very different plugins. First, you need a bridge to handle user table synchronization. You not only have to create a user for each site, you have to sync everything about both accounts each time anything changes. This would include changing email address, password, or even user name plus any other related data such as the use groups that the user belongs to. A good place to start would be jFusion, which is designed to bridge Joomla with other software packages but can certainly be modified to bridge Joomla sites. JFusion also has the added advantage of creating user sessions for each connected software package for a single unified login, which would be the second piece of the puzzle you would need to solve once you get the users sorted out.
http://www.jfusion.org/