Yep, they hurt. As noted above, aoe stuns are counterintuitively lethal. If you've got anything that scales with attack power, wait a few seconds before hitting it. You'll have 150k AP or so thanks to vengeance, so any scaling shields and heals get downright silly. Make sure you're rotating all the external cooldowns you've got as well, and that they aren't being layered with the tank CDs.
If one tank reliably goes down sooner than the other, what you could try doing is to have the entire raid blow up the windblades on the squishier guy while the other one just toughs it out or kites for a while, and then have the first tank taunt an add or two off the second.
In my experience as a brewmaster, my DK co-tank kept dying first, so we threw all the dps at his pack while a healer and I handle the other one ourselves. Fort brew, elusive brew, and a crit trinket get popped on the pull, and I also throw Xuen at a windblade to keep it busy for a while. Guard soaks a good 600k damage once vengeance ramps up, and the crit trinket stacks elusive brew back up quickly. By the time I'm actually out of cooldowns I've already killed one windblade, another's likely to be wandering around fixated on someone, and the dps have cleared out the other pack entirely.