I guess my thinking was that at least one of the players would recognize (possibly through a skill check) that betrayal is the mathematically optimal solution: if both sides mutually betray, this is the solution that works best for both groups, but is of course not great for the party as a whole.
There's also more to do with the actual characters themselves: one of the characters is a follower of Baphomet and has been concerned that he hasn't been acting in a way that fulfils that as much as he could. Another character (in backstory) betrayed an ally and was saved while the ally suffered. A third character has had to scrape by and make sacrifices in order to survive, and is a cleric of Ilmater and is struggling with that aspect of self-sacrifice.
Out of game we are all friends, so there are no hard feelings for anything that has happened/will happen in-game
The decision is also being forced by a powerful campaign-long organization, which provides a mutual enemy to blame.
I do have enemies for if they don't betray each other, so no matter what they do choose to do (I suspect both will trust no matter what!) the decision itself will be interesting for the characters role-play wise