Splitting a party can work if everyone controls two characters, and you split up so that everyone gets a character in each group. But sometimes it makes sense to split the group along a different axis.
Or you can claim it makes sense to do so, just to annoy your GM.
Commentary by memnarch (who has not seen the movie)
Well, uh, okay then. I guess Finn is just getting left behind for now. Aaaaaand so is most of the rest of the group. I guess that makes my guess for what happens in the comic more likely for the movie then. But if the rest is staying behind for Resistance reasons, then what's going to bring them all back together? Distress call from the Resistance or a resupply needed for the Falcon perhaps?
At least Chewbacca gets to stay with the Falcon. That's pretty close to having it left to his care. And frankly, it'll probably have better care now that Rey owns it instead of one of Jim's characters. Pete might like to kill lots of things, but he's not suicidally over-confident like Jim is when fighting.
Transcript
Leia: This seems like a good place to end for tonight. We can start the quest to Ahch-To next time.
GM: Actually, this is kind of the end of this whole adventure. Except I have one small thing to add, which we can do if you set out now.
Rey: All right, let’s see. I’ll pilot the Falcon, obviously. Ben is Chewie. Corey is BB-8. Sally, C-3PO. Jim is Allan.
Rey: With Finn out, Annie, you better be Leia.
Leia: I can’t come with you, Rey. I’m the leader of the Resistance. My job is here.
Allan: Yeah, I better stay too. If the First Order tries something while you’re gone, I’ll kick their butts!
BB-8: You’ll need me in the droid seat of your X-wing.
C-3PO: And I should stay to look after Finn. And... everyone else.
Rey: Right then. If I take R2-D2, the expedition is two-thirds me. That should be pretty good XP-wise.
Chewbacca: That is not a legitimate reason to take R2.
Rey: And I outvote you on any decisions!