Yes it's normal. There are a lot of threads dealing with this but in short what I have found is:
-On startup one core will run in 3d mode and one in 2d mode. Once you have been into and exited a 3d app it will properly return both to 2d mode to save power.
-One core will always be hotter, it is cooled by hot air already passed over the first core.
-Default fan speed is too low,(opinion) most of us seem to agree that 35-40 is the best compromise between lower temps and sound. I run at 40% fan speed and idle at around 50 instead of 80 and hit around 75 in games.
I have the same problem with 4870x2 in Windows XP only, so I dont think this is normal. When I run 3D-Mark in XP, I get half the score as comparing with running on Windows Vista. In Windows Vista, both adapters are enabled even when idling at start up. I an still searching for an answer.