It depends where the other group is, but I think you might experience more trouble than you're anticipating. Those Banshees can spot Marines from a long way off; and each time one veers off from you, it can get quite distant, which could make that other group a more attractive target if spotted. However, by all means give it a try.

: In any case, I need somewhere to put the other
: half of the Marines - killing them will provoke the anger of the others,
: letting them stay will stop the Banshees from spawning (if I remember
: correctly)

I'm not aware of anything stopping the Banshees from spawning. If you want to hide some Marines from the Banshees, one place you could try is the stream tunnel outside the rockslide area.

: The only problem is
: that if I head to the end of the stream, the Banshees may not follow
: entirely; but if that happens, I can just return to the checkpoint and try
: again. At least, this is what I would assume.

I'd expect them to usually follow.