1) Bump rule here is 6 hours minimum..
2) trying to intergrate 2 databases (and carry out a check on the second to verify username) would be a complex and dirty task.
If you want to have 2 sites/sections with 2 different databases but just the one user base then it is best to sort that out when you begin!
However, the answer to you question would be to use the forum database and user tables for both your databases...
Best asking for help in MOD request and checking the MODs section for possible Modifications that maybe of use.
Checking with the software writers of the other database/software package if the is a means to share the phpBB3 user tables.
Des. . .