Keep your tanks stationary. There are very few mechanics in the fight that require you to move. The exception is melee moving out of AOE/rocks, and of course, the red circle phase. Your tanks should never have to move if positioned correctly.
Actually, that's not strictly true. You need to keep the two bosses more than 25 meters apart, otherwise Fearful becomes a problem with rock throwing. However, the maximum range on single-target taunt is 30 meters. That's…very tight. I think it's definitely possible to have positioning like that, but I need to take a bit of time after the next time we clear that boss and measure a few distances in the arena. Our current positioning has the bosses nearly 45 meters apart, which creates a substantial delay when target swapping due to the time it takes the tanks to run 15 meters.