You're doing it right. The monster are basically doing an Australian Peel (technically an Australian Peel on a two-man front). The only issue might be if you don't allow the players the same opportunity if the tables are turned. If they are fighting a BBEG and want to use the same tactic, you have to let them if you're going to be fair (they could all declare they're going to act in concert on the lowest PC's initiative roll or they just choose a player to roll for the whole group).
They do have a minor legitimate gripe in the matter of fairness because they are more likely to face hordes of weaker creatures and benefit less from the tactic, but that really won't matter once they get access to fireball et al.
They do have a minor legitimate gripe in the matter of fairness because they are more likely to face hordes of weaker creatures and benefit less from the tactic, but that really won't matter once they get access to fireball et al.