Can PostgreSQL slave server act as master for a new database?
You have a few options. You could use DBLINK for your queries to run them on slave and get the data over to DB2 on master.  You could also do that with FDW's set up in DB2 on master.

The other option which sounds more like what you want in your title would require you running 2 separate Postgres clusters on the machines.  So you'd have two separate installs, 2 different data directories, running on different ports. DB2 would live on a different cluster than DB1.  You could set up your streaming replication differently for each of those databases at that point (where as if they're on the same cluster, you can't pick and choose).

Hope that helps.