I'm not sure what you mean with the "lemming width" separation, but basically what's happening (I guess) is that lemmings always update in a certain order, and the bashing lemming is somewhere in the middle of the list of lemmings kept by the game. So the first few lemmings move, with the wall still intact, and turn around. Then it's the basher's turn, and he breaks the last bit of wall. All further lemmings will now not turn back. In the case of a single pixel gap, I would expect this to lead to an offset of 4 pixels, since lemmings actually enter the wall by one pixel on each side before turning around (so one group of lemmings will need to go back two pixels, while the other group moves forward two pixels).
It could become more complex if the basher bashes "through" the gap, breaking the wall on both sides in one go. Not sure what the distance between lemmings would be then.
If you ensure that the basher is the first lemming out, the group should stay intact. Taking the second lemming out as the basher should split off one lemming, etc.
Note that I didn't try any of this out, it's just what would happen in my Pocket Lemmings clone (which hopefully models the real game...)
Cheers,
Zen