Could it be that the "drier" animal is about to shed? Other than that I'm not sure what you could mean by "cracks" in the skin. Perhaps a picture would help.
If it's not shedding - there are several other issues that may be affecting just the one skink and not the other that may affect skin condition. I've had the unfortunate luck (can that really be a phrase?) of dealing with the same situation here. I have a male PTS that is housed with 2 females. They shed for me beatifully every time, but he always has issues and is short several toes because of it. I've had him checked repeatedly for parasites (none), and his health seems generally to be good otherwise (so not an obvious illness). He eats well and eats as much as the girls (not a diet issue). He isn't stressed socially (he is in fact the dominant animal in the group)or environmentally as far as I can tell (not stress-related). Vets that I've spoken to suggest that metabolically there may be some issue, but it just isn't worth trying to pin down if I can manage the shedding issue through regular soaks.
If I were you I would examine all of the above issues for your skink. And if nothing seems to present itself as a likely culprit you'll have to decide how far it's worthwhile for you to pursue the issue medically.