"Unlike other fighting games, in the 2D Mortal Kombat games, both players cannot be hit at the exact same time. One of the players has priority over the other. Which player has priority is selected at the beginning of each round. If two people pick the same character and are near each other and both players do a HP at the exact same time, one player will take priority. Which player has priority stays the same for the entire round.
This applies to projectiles as well. It can appear during normal gameplay that both players can be hit by projectiles at the exact same time. In fact, their life will even begin to drain on the same frame. However, one player was actually hit before the other. If both players are in danger and use a projectile so that they'll connect at the exact same time, the player with priorities will hit first, kill the opponent and he or she will survive."
Taken from
http://wiki.shoryuken.com/Ultimate_Mortal_Kombat_3
