Yep. Since dd2 skipped lvl 5 (competed one meet and moved up), she and dd1 were both 6s for one season. Last year, although not the same lvl, they worked out together on the optional team. I used to ask the coach never to put them as partners for any drills. My favorite used to be when they'd tell what the other was "thinking." Yeah, like you know.
I know it's as annoying as all get out, and if it isn't over gym skills, it's over who ate the last of the breakfast cereal, who got the "end piece" of the banana bread, who's in whose spot on the couch, who changed the channel, who sings better . . .
I can't help you because I never got mine to stop fighting. One of the things that compels them to excell in sport is quite a bit of inborn competativeness. I'm surprised, though, at how often, if I lay low, the fights work themselves out. Even if one marches off to her room, by the time she comes out, things seem to have blown over--or at least they move on to a new battle. If one wants to "talk" at bedtime or something, I just give them sympathy. They seem ok with that.