Uh, no. One tank is MT so you're comparing the DPS gain for him being in the DPS stance (20% gain) vs any healer DPSing (the other tank is in DPS stance by default so nothing changes except for adds or if they design more fights where both tanks tank most of the time). You can alternate who DoTs and who heals MT so you need to factor both healers in so the total uptime gain isn't 60%, it's only some amount between 20 and 30% for both combined or like 10% for each healer.
The problem is not that you can't all DPS, the problem is that you can. If healer DPS uptime is 0% and then turning on your tank stance lets the healers trade off applying DoTs, it's better to turn on the tank stance for DPS. The problem is right now healer DPS uptime is like almost 100% during the early phases so you gain little uptime for turning on your tank stance and that uptime is probably a low potency filler spell which can miss.
If you adjusted incoming tank damage so that it's almost impossible for healers to DPS, healer DPS will be higher priority than tank. It doesn't matter that the tank has 100% uptime, if the Scholar can apply Bio II and Bio every 30 seconds and the WHM can apply Aero III and Aero II every 30 seconds that would likely be better than the gain from turning off Grit 100% of the time (even more so for AST, since Combust can't miss and is quite high potency).