In order to limit confusion and possible shutting down of "old" DB server by uninformed DBA's, you should consolidate all project schemas/databases into the SAME DB server, if possible.
If you were using DB1 at the time this instance of ALM was installed, it very likely also has the Site Admin schema (sometimes named "qcsiteadmin_db") there and pointed to by the ALM service as it starts up ALM.
If you de-commission DB1, you should migrate your Site Admin schema (as well as all project schemas) to the newer DB2 (and run the stored procedure to "fix" the td user - if using SQL-server with SQL-auth).
You CAN re-point where ALM gets it's qcsiteadmin_db from to use DB2.
Also, if you need to move the project schemas from DB1 to DB2, you follow the usual Remove/Edit DBID/Restore process. Here, you need to edit the DBID for each project that was using DB1 to have the same DB server name and URL as one of the DBID's for a project using DB2.