It can be frustrating if you maintain two or more accounts on GTalk especially when there are no overlaps and each account has a different set of friends.
You need to log off Account A in order to chat with friends of Account B and vice-versa.
I face this problem all the time because my old Google Talk ID, that has most friends, is associated with a Gmail account while the newer one is on Google Apps.
Though there are hacks that allow you to run multiple instances of Google Talk simultaneously using the nomutex paramter with gtalk.exe, a much better solution is Disgby.
With Disgby, you can chat with multiple Gmail IDs simultaneously in one client. You can even switch to offline or invisible modes, a privilege currently not available in the standard Google Talk client.
Best of all, you can use the same Disgby client to read Facebook messages and Twitter updates. Highly recommended.