I've worn a 66" sword on my back in a LARP. It gets in the way a bit but I don't think it's that bad. Could be an issue crawling through tunnels and you'll have to duck entering some doorways etc. You can do a simple strap with a ring at each end, it's perfectly fine for carrying but you have to take the whole thing off rather than drawing. I don't know what else was used historically, but in practice, quickdrawing was probably never a thing for two-handed swords. If someone was jumping you in the street, they would probably be attacking from inside your reach and you'd be dead anyway. Pull a knife instead. On the battlefield, you'd already have the sword in your hand.
Still, for gaming we postulate heroes carrying around two-handers on a routine basis, so...
Back sheaths are kind of inconvenient to draw from. You certainly could not draw a two-handed sword over your shoulder from a regular type sheath that is the full length of the blade, your arms are not long enough (without grasping the blade, which is possible but shifting grip partway through will make it slow). But a lot of two-handed swords have a ricasso that wouldn't fit in that type of sheath anyway. If the sheath is only half the length of the blade, drawing is fine, you just need to add something closer to the hilt to keep it from swinging around too much, smacking the back of your head and not being in the right place when you need it. Could just be a cord tied with a bow at your shoulder, pull the string to untie, and then draw.
The best arrangement I used had a two-part closure that fit sort of like a buckle, just below the hilt of the sword, but held together with a strap attached to the sword. So, as soon as the sword was drawn more than a few inches, the buckle released, completely freeing the sword. It drew very easily, just required some fiddling to sheath again.But that's ok, sheathing your weapon after the battle is not the part that has to be quick! I'm probably not explaining this very well but it was all done with leather and rivets, nothing magical, magnetic, or modern.
I think it's reasonable to imagine that people who routinely wear two-handed swords and get jumped by giant monsters could figure out an arrangement that would work.