Either or both creatures can be tapped prior to using the ability and it still works.

The power of the creatures when the effect resolves is used to determine how much damage is done.

In multiplayer games, you can choose a different opposing player each time it the ability is activated.

If either target is illegal at the time the ability resolves (whether because it has left the battlefield, because it has gained Shroud or Protection, or for any other reason), the ability taps the remaining target, but no damage is dealt. If both targets are illegal, the ability doesn't resolve.