You can't do it with just two diggers on Genesis because you can't get the second lemming to reach the first in time.
Actually, if I recall correctly, you can do it with 2 diggers on Genesis, if you pause immediately when the level fades in, and then immediately prepare to raise the RR as much as you can when you unpause, before the first lemming enters. I mentioned this at the old old forums (around the same posts where the discovery of the trick we're talking about was made). If you want proof, I could go about and see if the emulator can record videos or similar.
On the other hand, the Amiga and SNES versions' RR-change speeds aren't fast enough for a 2-digger solution. Amiga is actually pretty close, I think the 2nd digger ends up coming out 1 or 2 frames too late. (SNES is not close at all.)