Quicksilver will not be distinguishable between Avengers 2 and DOFP
Think about it, we're up in arms over the prospect of the general audience being confused by the character being in both films, but I don't even think they will be able to connect the two. They will be completely different and have entirely different sized roles between the two films. The only similarity will be their powers. In DOFP, the character will be called Quicksilver, will only be used for one action scene, will have a completely different actor, will be American, and given how small his role is the general audience might not even remember the character much at all after the film.
However, in Avengers 2, the character will be known as Pietro, he will have an integral and large role in the film, he will be british, and the source of his powers will be completely different and fleshed out.
The only discernable similarity between the two characters for the general audience will be their powers. Otherwise they will be completely different as actual concepts. I don't see think this will confuse anyone and I doubt a lot of people will be able to make any connection between the two films.