I don't really see this as a problem. Realistically, multiple observers usually do make it more likely that one of them will notice something wrong. And a 5 point advantage is not nearly enough to guarantee success when you're rolling d20's--the more skilled character's mistakes will occasionally be caught by others.
There are places in the rules where it suggests that in many opposed check situations, dice-rolling can be reduced by assuming that either the active or passive skill user will take 10 on the check--using Sleight of Hand to conceal objects, using Notice to detect intruders or see through a disguise, etc.
If you wish, you could apply this rule of thumb more generally to Narrator characters. For example, to sneak up on (or past) an orc patrol, each hero rolls Stealth against 10 + the orcs' Notice modifier (usually the same number, though a leader might have more ranks and thus occasionally be the only one to succeed). However, heroes should get the choice of rolling or taking 10 (if the latter is allowed in that situation). They will sometimes roll very high or very low, but that's part of the strange luck of being the central heroes of the story.