No way to prevent chipping except to stop bashing your skis together. Best thing to do is to just accept that its gonna happen and that it doesnt really affect the skis performance other than change the aesthetics.
Once the entire topsheet is chipped off. MUAUAHAUAHA
But for real, it will happen no matter what ski you have and how you treat it; they will get well-chipped if you use them for any significant amount of time.