In MapGuide, once you have connections to both the spatial and non-spatial databases, you can do a join, which should allow you to do what you're looking for.
For example, go into the connection setting for the spatial connection. You'll see the last section of that page called "Add properties to classes using joins and calculations". Create a new Join. It may take a while to scan your databases, but that's normal, don't be alarmed. Then you can join data from 2 different sources on a particular key, which will essentially create a new "table" that you will be able to create a layer off of.