I wouldn't worry to much. My ones have and still are doing the same. I wouldn't separate them as piggies are social animals and if they are used to each other its not fair. Though I would separate them if they really start getting vicious, so like causing blood. What I do is take one out if they are fighting especially when there is squealing and give that one (normally the little one) more attention. Its probably the big one trying gets its dominance. My ones also used to mount each other, both girls though. Also make sure that there is nothin that they can get proper dominant about, like a toy or house bit, my one got really vicious when we put a mini house thing in there cage, the big one wouldn't even let the lil one step in.
So don't wouldn't wrry until it gets like really bad, cause its probably just dominance. You'll probably make it worse if you separating them.
xxxxxxxxx Hope I helped. xxxxxxxxx
!!!Owner of two piggies who used to fight a lot!!!