When our heroes are sent into the fuselage - in the order ‘Arthur, Douglas, Carolyn, Martin’ - they form a ring, so that when Martin collapses, Carolyn is standing directly in front of him and would only need to turn around on the spot to rescue him. Similarly, Douglas is standing directly behind Arthur and probably felt him go down. Therefore the most natural course of action, based on their relative positions and the difficulty of locating bodies by sight in a smoke-filled cabin, is for Carolyn to rescue Martin and Douglas to rescue Arthur.

It’s also worth noting that Carolyn is rather smaller and older than Douglas, and it’s therefore not unreasonable to imagine that she has more difficulty lugging a body around than he does. We know Martin’s decidedly titchy; but if Arthur looks anything like Mr Finnemore then he’s tall, and if all that quiche goes anywhere then he’s not exactly a stick, either. Oh, look, here’s a crappy diagram in case I’m not making myself clear:

It’s impractical for Carolyn to rescue Arthur. It’s impractical for Douglas to rescue Martin. But they do so anyway. Personally, I like to think this has as much to do with Douglas’ paternal instincts as it does Carolyn’s maternal ones.