Mark CMG
Creative Mountain Games
(. . .) they've both said that they won't accept reforming the group during this campaign, pretty much regardless of the situation.
Ultimately this is your call as the GM. Sometimes players will have characters do things because they believe that the GM or the others players will simply have to accept it, because the other players won't boot them, or the GM won't scrap the campaign that has taken a long time to devise, or they believe that they are somehow indispensible (like everyone is playing at their house). A GM has to be careful not to let the tail wag the dog.
In the type of campaign you are running, you need the players to create characters that form a cohesive party, one group, with their goals aligned with one another. Tell the players this is what you require for this campaign and they can either adjust these characters or create new characters and set these aside for the time being to be picked up later for a different campaign. If they cannot do one of those two things, then this is actually a player problem and you will need to decide what changes to make in that department. If the players don't understand the first part regarding the characters, you can either let them know that you will have to make player changes and see if that helps them understand how serious this is to you, thus moving them toward making the necessary adjustments, or just form a seperate group unilaterally to run this campaign and keep going with the current characters in whatever direction they go.
It might be fun to watch them destroy one another. I once had a group so bent on PC vs PC conflict that they didn't realize I had adjusted the setting to be villain, monster, and practically conflict-free (not too tough as it was mainly a city campaign). I ran for three sessions before one of the players mentioned to the others that they hadn't been in a combat outside of the party for a while. The five player party had gone through nearly a dozen PCs by this point. I allowed new PCs in at the level of the group which was fifth then sixth after experience from PC kills. Yes, they actually leveled during this three week period and even chuckled at the idea that some experience came from PC deaths. When I explained what I had done, the two biggest instigators left the group and a new core group emerged that was very tight-knit going forward.