One is good, two or a modified single better for a trials tire. The tube shifts from its own inertia, not the tire slipping. Mark the tire and rim and you will see. This is very common with the BStone UHD tubes. The Moose HD tubes seem to be the best for durability and to avoid shifting.
Mich X-11 has a very weak sidewall, not the best choice for Tubliss. They even tear up with tubes. MT-43 is very good Tubliss. I will try another Mitas as soon as they are available again. These two have tougher sidewalls. Dunlop is next best choice.