Except the only problem with that is, once one of your teams is KO'd, you're either left with a ridiculously powerful team that can kill a regular boss in three or four hits, or an underleveled team that will get killed in one or two hits by the hard Mark Hunts and secret bosses.
You might have thought that you can use your second party to revive the first one. Except the problem again, is that your uber strong party is almost invincible compared to the bosses in the story, making a lot easier than it should be to revive your characters. And your second party still has the problem of being too weak for the secret bosses, making it hard to revive your other party before they easily die. So actually, what you got was a mediocre, split in half, version of "both worlds."