A very long time ago World of Warcraft designed around the idea of a main tank (Warrior) and off tank (Paladin) and they eventually learned that such a design is untenable. However, I do want to add that (in the current savage mode content of which I’ve cleared everything) you can clear with any tank combination.

But as things stand in the current meta, the warrior MT and Paladin OT just have way too much synergy with each other including the way their cooldowns line up with certain boss mechanics. I believe that is the situation that needs to be addressed first if they want to make the tanks more balanced.