Have one drummer play a short beat or lick that the other drummer will try to mimic. This beat should be no more than a few seconds long, unless you want to raise the level of difficulty.
The second drummer tries to play the same beat or lick. If unsuccessful, that drummer would receive an "H."
If the second drummer cannot play it without messing up and gets the "H," the first drummer will have another chance to play something the second drummer might not be able to mimic. That would result in the second drummer receiving an "O," and so on.
If the second drummer is successful in mimicking the beat or lick, he would continue back and forth until the first drummer messed up in trying to play something for the second drummer to mess up on. Then the second drummer would have a chance to play something that the first drummer would have to mimic.
Whoever receives all of the letters, "H-O-R-S-E," first will lose the game and all bragging rights.