You don't have to kill them at the same time, actually. I just tried it, and it seems a Gargoyle is revived about 10 seconds after dying, unless both Gargoyles are dead. So just cast Golem at the beginning of the battle, keep attacking the statues equally and when you get one of them killed, kill the other one quickly so that the first one won't have time to get revived.

EDIT: If you aren't fast enough to kill the 2nd Gargoyle after killing the 1st one, use Haste2 and/or set Battle Mode from ATB to Wait. I think Monks are good against Gargoyles thanks to their double hit and Kick ability.