In an active-passive topology a WebLogic Managed Server can run on two available systems, however the Managed Server does not runs on both systems at the time; The Managed Server uses the other system to migrating when occurs a fail in the first system. In an active-active topology there are usually two WebLogic Managed Server running on each system.
You can implement an active-passive topology using WebLogic Server Whole Server Migration. Go to http://download.oracle.com/docs/cd/E12839_01/core.1111/e10106/aa.htm#CHDICJCG.
I hope this helps.