What if you have two roughly equal thieves (in the right type of game, you could have two sneaks, a bruiser a face and a mastermind) and they are both sneaking in... which one is the assistant? You could do some narrative hand waving, after all if they are about equal than their times helping and times leading will balance each other out and be roughly equally effective.

So you can work it. However it fits into some other design issues I'm having and I'm curious because it never seems to have been done before. And I kind of find that hard to believe.