[SixOfSpades]

Because you run into M&X and K&J so early in the game, they don't really have real NPC quests, the way some of the other NPCs do. BioWare felt that throwing a large number of subquests at a newbie just staring their maiden game of BG1 would be needlessly confusing, so all they did was have the two pairs of NPCs do their part to push you along the Main Plot.
M&X: We are on our way to Nashkel. We won't mind a few little detours with you first, but we have a job to do in Nashkel.
J&K: Oh, good, you're here. After we chat about Gorion, we should get going, because we have an appointment in Nashkel.

In either case, as soon as you speak to the Mayor of Nashkel (Berrun Ghastkill), then their NPC "quests" are done. If any of those 4 NPCs are in your party, Ghastkill will say that "I don't know you, but I recognize ? in your party."

Important note: If you should pass by M&X or K&J (not recuit them at all) then enter Nashkel, and then go back to where you left the NPCs, they will be gone permanently. Just a little warning.

And it IS possible to keep the conflicting pairs in your party for a long time--it depends on Charisma, and keeping your Reputation at a level that all 4 of them can live with. Also, it seems that the 4 are less likely to start a fight if some of them are missing (dead or "misplaced"). In my last game of BG1, I kept Jaheira and Montaron in the party all the way through the end of the game. Khalid and Xzar, on the other hand, were quickly abandoned in some out-of-the-way house. Now, my main character's Charisma was anything but spectacular, so I suspect the 4 are unwilling to tangle with each other if they've already lost somebody.