App.config And Connectionstrings With Several Databases

Hello all

I need to know the best way to work with databases (Sql Server) in my app.config.

I have an VB application that binds data from a SQL server (All Customers install these application and retrieve data via VPN). I use two databases for each customer. The first DB has some commun tables/data and the second one is its ID (Until now I setup manually the connections because we don't have more that 10 Customers but we need improve that for next year). I know that a connection string is created in the app.config file . 

I don't know if it is possible from this way or if there is another way to do it.... but the goal is:

1. Customer is logged in the application -User Name, ID, Password

2. Customer retrieve a database with commun data and his own database (that is his ID). (From the app.config???) How can I setup my app.config in order that every customer retrieve his database automatically when it is logged?

Thaks, I don't know if it's clear but I didn't find another way to explain my trouble