They do seem to know when a lamb has something wrong with it or is weak - and nature being fairly blunt about these things, there's no point putting energy into something that's going to die, when you might as well feed the one that will live.
Shearlings are also a bit daft sometimes about realising that the lamb is something to do with them
I do as you do with the second one, get it feeding without being knocked about. Usually once it's begun to smell of her milk, she'll decide it's ok.
I make a 'corner' in the pen, with a barrier high enough for the lamb (and its sibling if there is one) to be kept in and the ewe not to be able to headbutt, but low enough for her to see and sniff. I let the lambs out often and supervise feeding, occasionally tieing the ewe up at the head if she's being especially bolshy. It usually sorts out in a couple of days.