Don't sheathed weapons have a seperate model?
So technically you could just make the sheathed Katana to be only the shaft of the Lightsabre. Unless I'm wrong
They have a separate model but that remains there, so it's either
a) have a katana for some reason then pull a lightsaber out of it
b) have 2 lightsabers, one that you pull out of your ass and the other one that you don't use
c) pull a lightsaber out of thin air