Often I hear that folks from two lines of the same sword school will have trouble practicing together, that it is dangerous because one of them is likely to move in a way not expected by the other.
Sort of like putting a Toyota crankshaft in an Audi engine I guess. But kata aren’t machines and we aren’t robots. The very fact that two lines of practice do the kata in two different ways means that the kata are not immutable things, they are capable of being done in more than one way. Unless of course you assume one way is wrong and the other correct. But if you assumed that, surely you would not be practicing with the wrong folks would you? Except maybe to prove them wrong? With the assumption that kata are then, not cars, perhaps we can find a way that one can fit two different students into one practice, at least for the short term while they practice together. “Oh, you cut in that angle, OK go ahead and I can block this way instead of my usual and we can go on from there.”
Pretty much all budo practice is involved with paying attention to what’s happening, at least as far as I can figure. If you’re paying attention to your partner during a defined partner practice, and he moves his weapon in a different angle than you expect, perhaps you can adapt to that? If you are both just doing your memorized dance steps as fast and hard as you can, I can see how one would get cracked over the head if the other moves in an unexpected way. But seriously, what about when your buddy in the same class forgets which kata and starts doing another one? Sure, same crack on the head.
Going back to the idea that one way to do a kata is correct and one wrong, I think we get to the real problem with practicing cross-line. Folks that are “doing it wrong” will need correction yes? And your way is so superior that you dare not modify the practice in any way.
Competitive stubbornness will certainly get someone’s fingers broke.