Congrats, you have two of the best healers in Eorzea :), given your clear is pre-echo :)
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Hmm I can hardly call it a problem. The transition won't take even 50% of the tank's health, still very capable of surviving a plummet. Heck, they can probably soak up another plummet! Chest bumping Twintania is something a tank can easily handle. Jokes aside, it's completely fine taking a bit of damage during/shortly after the transition. Whatever happens in that phase is either dodged or will instantly murder you, tank or no tank. A single regen from the white mage or some fairy oil massage* will get you topped off before the phase is over.
*I pray all of you have done the Manderville quests...