I have not done the the DB Control for more than one instance, but I
would expect that you should be able to do it.
If there is no "sysman" account, you need to manually install the DB control by setting up your environment and running "emca"; it will prompt you for everything it needs.
If you already have a sysman account and its not working, you should try to reconfigure it, which means rebuilding the repository. I had to do that after simply changing the "domain", as its connection got out of sync. In the case of multiple instances you need to make sure each EM has its own set of ports, so I'd suggest reconfiguring the 2nd instance while the first is up and running.
Here's what I did to "reconfigure" EM (I believe I found this on metalink somewhere):
emca -deconfig dbcontrol db -repos drop
emca -config dbcontrol db -repos create
You can do a "emctl status dbconsole" to see what port it is running on; as it seems to grab whatever it finds available.
And BTW, you will need to start the EM at bootup via: emctl start dbconsole