Are they of different sizes and small still? There is a chance that if they are both young and remain the only clownfish in the tank that they may get along.
The best way to tell the difference between the 2 is the eyes. Both have black centres to the eyes. Percula's outside of the eyes are then orange, the Occelaris outside are merky orangish black/brown.
Occelaris:
http://redcoralaquarium.net/online/i...roducts_id=410
Percula:
http://redcoralaquarium.net/online/i...roducts_id=411