Misinformation is all over the place whenever people talk about this fight. Mostly because the fight hinges around the dps speed, so if you get used to doing it with a static group - you're in for a different experience if you get in with another group with higher or lower dps.

Suparna loses wicked wheel after Chirada dies, so she's actually safest to double tank. Chirada does not lose downburst after Suparna dies, and it hits like wicked wheel, so she is more dangerous to double tank. In a decent party, this doesn't technically matter either way, as the first sister will be dead and the double tank situation will be over before the 2nd supermove comes out. But we aren't talking about decent parties, we are talking about random guaranteed terrible FFXIV players.

If your party simply must kill Suparna first, just make sure you don't dps Chirada at all until she's gone. A not very fun way to wipe is to have MT holding Chirada and Garuda, and then Suparna and Garuda switch places. Sup and Chi get their 8 stacks of invulnerability for standing next to each other, and the party is left wondering wtf happened.