Share centralized queries for multiple databases
Some other ideas:


Use readily available tools to compare two databases after a change is deployed (you can automate this perhaps, depending on your existing deployment process), and synchronize them
Automate the deployment of changes such that changes to procedures always occur against both databases (again this depends on what your current deployment process entails)
Implement dynamic SQL carefully such that data is pulled from the right database depending on the user connecting.
Use a central database with views or synonyms under different schemas, each set pointing to the two different databases. Construct the calls to procedures dynamically, or leave off the schema prefix and set the default schema per user (you can double up on the security by granting database A's user access to B's schema and vice versa).