If I have a friend with X and I have Y, and we switch games and beat them both again so we get the legendaries etc, is there a way to trade those legendaries to ourselves so each of us winds up with both Yveltal and Xerneas?

I've heard someone suggest just trading it and then switching the game off once the trade is complete without saving, but the game saves automatically after a trade is done, and if you switch it off mid-trade it'll just cancel the trade and each of you winds up with just the mons you had before you started the trade.