|Easier said than done?|
Two players face each other. One has two balls. One ball is tossed to the other player underhand. The other is tossed overhand. This occurs more or less simultaneously. Naturally the receiving player is expected to catch each ball. Receiving player then becomes tossing player. Lather, rinse, repeat.
Sounds simple enough until you try it. If it is indeed too simple for you, try the progression of moving sideways across the court while you are tossing.