Players do that sometimes, because a single tank with 8 stacks of Vulnerability Up can be killed sometimes, very rare but it does happens, so some tanks prefer to have swapping at 3-4 stacks. Before Coil 1 was put on DF, players set up PF party instead so they can just set it to single tank, and you don't need 3 healers, 2 is fine, so before DF you will never see players forming T2 enrage farm with 2 tanks. With DF you don't really have much of a choice since you definitely will get 2 tank.

But based on what you said they are doing it wrong, you do not stand next to each other for tank swapping, one will stand infront of ADS while the other 90° from the front to prevent both getting cleave. If there are no other melee beside the tank then they can just stand opposite of each other. At 3-4 stacks the OT will provoke from MT, MT will stop attacking for a while so that the OT can gain agro.