This is an interesting point you raise.
I am implementing at a client site, one of the requests was to use a single database for both VC and UM. Was my first 3.5U2 deployment, and I didn't know if it could be done. We are installing on an Oracle DB rather than SQL, and during the installation of VC, installing VC and UM in the same database is the default option - ie: you don't get a chance to point to a different database for UM, just the one ODBC DSN to point to during installation.
Interesting, as I asked VMware about the option of using a single database (and so did the client) and we were both told "No".