Healers, thoughts on the dual DPS tanks?

[Bourne_Endeavor]

1.) Not being told beforehand, or worse, one will say they're MT but switch soon after

This is standard practice for all tanks. Ideally, you want to drop tank stance once you've established enough of an aggro lead. Assuming proper swaps and/or Shirk usage, there is no benefit whatsoever using tank stance unless things begin falling apart. Mechanics even at the Savage level simply aren't threatening enough to warrant "stance swapping." Hence why tanks prioritize damage.

2.) People aren't always amazing at dodging so now I'm pressed on time to spread heals when I wouldn't be otherwise

You shouldn't have any issues healing other players if the tank utilize proper CD management. If DPS continuously take avoidable damage, the blame remains on them and one shouldn't be demanding the tank adjust because someone else refuses to do mechanics.

3.) Their gear is just too low to do this comfortably

Gear disparity generally impacts aggro more than incoming damage, assuming the tanks know how to mitigate. Tide Pod will only hurt slightly more if I'm in i350 gear attempting Ridorana compared to i370. I will, however, need to do more aggro management if the DPS are good.

4.) Not knowing right away who ends up taking the tankbuster

This should never be an issue. Whoever the boss targets is, for all intents and purposes, the "main tank."

5.) After a lull or break in the fight, I'm going to end up pulling aggro and now have to schedule my Lucid Dreaming for their sake (esp. when I'm WHM)

You should be doing this. Healers are just as responsible for their own aggro management as tanks. You want to use Lucid early to get it on cooldown, thus allowing more uses throughout the encounter or to cut your enmity. The only instance where a tank needs to go back into tank stance during a phase transition is if aggro resets, though Warrior's tend to do so because Unchained offsets the penalty by a fair margin.

6.) Some CDs are just not as effective at mitigating damage unless in Tank stance (I'm looking at you WARs)

There is not a single tank buster in the entire game where tank stance is required to survive. Ultimate notwithstanding. You will always have either a CD or enough party mitigation to counteract the incoming damage, unless the tank does not know how to manage their CDs. In such a scenario, they're either inexperienced or bad. Warrior, in particular, has a fantastic suite with Vengeance, Rampart, Thrill, Raw and Holmgang. Keep in mind, as HyoMin said, Defiance only increases their HP, but does not reduce the incoming damage.