GB,
You're the GM in an RMSS game, a bunch of PCs are locked inside metal boxes the size of a phone booth that can be opened with a key from the inside or outside. They have their usual gear with them, including light sources and tools suitable to lock picking with them.
#1 is a thief, has ranks in S:M and Lock Picking, and a +100.
#2 is a clockmaker, has ranks in Crafts, and Crafting: Clockwork devices, total bonus of +100. No ranks in S:M or Lockpicking.
#3 is a locksmith, has ranks in Technical Trade / Vocational and Locksmith, total bonus of +100. No ranks in S:M or Lockpicking.
Most GMs would allow #1 to pick their way out at +100, then let #2 and #3 attempt it at some penalty. . .likely #3 at less of a penalty than #2. Only the most hardcore "Rule as written" GMs would make #2 and #3 make their attempts at an effective bonus of -25 "no skill".
Would you make them both roll against -25 "no skill" or use your judgement, apply a penalty to their applicable skill and let them use it?
Moriarty,
I'd add another vector there.
1) Skills can share a "Theme" like being "Sneaky skills".
2) Skills can share aspects in being "Similar" in how they work.
3) Skills can share stats, like say Dancing and Martial Arts. . . on the assumption that practicing martial arts will strengthen your muscles and make you more flexible, which will make you a better dancer, and vice versa. Two skills that are completely unrelated in any other way, may be related by stat, which will make them share that aspect also.