YES....you can! I just stumbled on this by accident.
Sign out of the chat clients, then IM someone from their contact entry or from your conversation list. (even though you are signed off)....so make sure the upper right menu has the IM client you want (aim OR gchat).
This will automatically log you onto that service. It will send the IM. Then when you go to your buddy list, you'll notice that only half the green 'available' circle is lit up. and only those buddies are signed on.
if you switch to busy or available, it will sign on the other IM client.
This was present in WebOS 1.0.4