If you Assist someone with a Mechanics check, and you have Gearhead 2 (remove 2 Setback from Mechanics checks), do you remove 2 Setback from the check? What if the person you are Assisting has a rank of Gearhead as well? Do you remove three Setback?

I don't think there's an official ruling, but the suggestion by Sam Stewart courtesy of an older Order 66 podcast appearance is that those talents only apply to when you're the one making the check, and that if you're just providing assistance (skilled or otherwise), then your talents don't apply.

Personally, I generally allow talents that remove setback dice to be applied when providing assistance. It's not come up that two PCs have ranks in the same talent, but I'd probably allow them to stack as my personal take on things, even if it's not RAW or even really RAI.

I think I would agree partially with @Donovan Morningfire here, and allow the assisting character to add such talent effects to the mix. Part of the assist, narratively, might be them warning you to do something in a specific way (or NOT doing something in a specific way), and thus cancelling setback dice left and right. However, I would probably not stack the effects of the same talent; if both characters have the same talent, but one has a better effect, apply the better effect. A couple more rules seem to work that way, not stacking the same effect, keeping the best of the effects instead. If both characters have an applicable talent with different effects however (one removing a setback die, the other adding a boost die) I would allow both effects.

This way people may actually feel their characters to be competent, to matter in the ongoing checks to keep the stories moving.