Oooo...On the genesis it is possible to do any of the "We All Fall Down" levels with only 3 diggers.
A quick update for two other versions I've tried:
SNES: 3 diggers is possible. As for 2 diggers, not even close, the RR don't go up fast enough
Amiga: 3 diggers is possible. But intriguingly, as for 2 diggers, it is close but not quite, I think the RR needs to be just slightly higher. If the second lemming is 1 pixel closer to the first lemming, I think 2 diggers would've been possible on the Amiga, but I have no luck achieving it, so maybe 2 diggers is impossible on the Amiga.
So it looks like the PC/DOS version stands unique in that the solution fundamentally doesn't work.
Aside: the reason the 2-/3-digger solution works is also the reason why guest's 47/50 solution for Wild 15 will fail on the Amiga and Mac versions of ONML.