Four Horsemen is not really an issue for 2 paladin tanks. There are two reasons that I say this:
1. If your DPS doesn't suck, you're going to kill one of the horsemen before this taunt issue becomes a problem. Once that horseman is dead, the issue is gone.
2. Coordination over Vent (3, 2, 1, TAUNT!) does result in both paladins successfully taunting the other paladin's mob.