I don't have a problem with PCs taking 20 on disguise (or even hide for that matter) when disguising (or hiding) someone else.
In game terms, person A is dressing / kaing up person B to look different. Person A rolls, then makes a spot check. If he sees something wrong (a bit of hair undyed, a mis-applied fake scar, or some such) he tries again. Actually, I'd be tempted to let person A keep disguising until he beats 20+ his spot check.
If you don't allow taking 20, you will just force your players to roll again and again until they do well. . .
For example: We have a group of three PC's.
Sneaky the Rogue has a total spot of +2, and a total disguise of +10
Bob the fighter has nothing for spot or disguise
Woodsy the ranger has a spot of +4 and no disguise.
Sneaky and Woodsy make up Bob (Sneaky rolls and gets a 10, gets +2 from Woodsy's assistance = total disguise result of 22).
Woodsy and sneaky really check the work out for a while (Woodsy takes 20 on a spot and gets a +2 from Sneaky - generates a 26)
Woodsy sees a miskate and points it out to Sneaky. They try the disguise again (Sneaky rolls again, gets a 17, total of 29)
Woodsy and sneaky look for a long time again (same 26) and see nothing wrong.
Doesn't this sound like taking 20 to you ?
It's not like Disguise is an often used skill in my game, so I don't mind PCs finding a way to be more sure about it.
-Tatsu