This level pair has a rather interesting history.
The easier version (We'll Meet Again) can be solved using either exit alone, or using both. My intention was for the harder version (originally called Lemming Dilemma) to have only one solution, using the left exit. ccexplore found a backroute using the right exit, which I removed by expanding the gap to make it require 2 builders. ccexplore then found a very ingenious route to reach the right exit on this revised version, which I was happy to leave in as a challenge solution.
However... there are times, few and far between, when even ccexplore nods. When I came to remake this level, I noticed that the backroute was still possible. Also, I could improve ccexplore's clever solution to make it use only 3 diggers. Since the backroute required 4 to be possible within the time limit (yes, I'm afraid on this one the time limit is absolutely essential) I could eliminate it by giving only 3. This would also eliminate my original intended solution (which used all 5) but that was okay -- there was nothing especially elegant or even difficult about it. So I prepared to release a version of the level in which ccexplore's method was the only solution.
But wait... might there be a way to reach the left exit with only 3 diggers after all? Incredibly, I managed to get it to work; this is easily hard enough to belong in the Challenges topic, if this was an official level. So now, in the final version of the level, either exit can be reached but both are much harder than I originally intended. Unless, of course, there's a backroute that neither of us detected....