Tricky Multi-site config

[eluser]ccachor[/eluser]
So for the requirements of this new project I'm working on there will always be two databases (Users and XXX-Site). The reason for using multiple databases is because each client has inventory that is quite large, so putting it all into one table is a problem. So, instead each client will get their own database with a table for inventory, etc. The best way, I think, is to have the Users table include a field for the name of the second database it will use. There could be one user for multiple sites so that's the reason for that.

I have the multi-database config set up, but I'm just wondering how I would set the field in the config file to get that given database based on the query from the first database. It's difficult to explain I apologize. I've been looking at multi-database posts on the forum but I haven't come across one where the second database name changes dynamically!

Any help would greatly be appreciated