How to integrate with other open source systems?

Hello, all, I need to bother you all once again. Because of your efforts, my project is now going on very smoothly.
Now I installed WPMU (named site#1) with some plugins in Premium zone. And now we also installed another open source system(named site#2) in the server, which have a separate database, user tables.
Currently I plan to keep the two user tables unchanged, and make WPMU fully runned in server but disable the edit/register/update/delete function in site#2. The logic is:
(1)For site#2, no body can edit/register/update/delete user information in site#2.
(2)if a user edit/register/update/delete his information in WPMU, the user information will also be updated in the user table of site#2.
(3)when user login from WPMU, he/she can also be authorized to go site#2 without any additional login.

Can you give me any suggestions? Thank you for your time on my subject!