I suppose one of them is screwed clockwise, as usual, and ther other is screwed the other way, counter clockwise. But I don't know for sure, because I use Pentax K-mount...
Seriously, though... they have different flange focal distances:
- Canon EF-S mount, 44.00 mm
- Pentax K mount, 45.46 mm
- Nikon F-mount, 46.50 mm
It means, that through adapters you are can put a Nikon lens on the camerasa camera with smaller FFD, but not the other way (with Pentax or Canon lens for Nikon body you cannot focus to the infinity, or you have to use an adapter with corrective optics).