It's not uncommon for overgeared groups to two tank and tank all the things together now. It makes for some fun parses, despite often losing a person or two along the way. Is it what I would recommend for a first kill? No, not at all. Using someone's undergeared offspec or tank alt to do the usual split strat would almost certainly be easier than the chaos that occurs when you tank them together.
But, as far as tanking them together goes, the important thing is to be really aware of all of the abilities. Ashen walls should be placed in good locations (generally speaking parallel to the room or if you can tuck them into corners), melee have to be very very careful with foul geyser/the slimes dropping (and/or their tank should run out a bit so they aren't be dropped directly on melee), people need to not eat iron tomb, tornadoes, or foul stream (other than the person it's on), and strong cds should be used for every falling ash (a disc priest can spirit shell every other one which is huge). People should be proactive about using personals or calling for externals for iron prison falling off it's aligned with any other debuffs or raid damage (again disc priest shield is huge here for the shield on every iron prison). I would still tank the bosses in roughly the same general area so healers can be in range of both tanks, and I'd probably three heal. Again though, this is all a higher level of play and awareness than is needed for doing any form of the split strat, and it specifically makes the fight quite hard on melee.
This is from a 10H standpoint, I haven't done the all together strat in 25H (only the usual split strat) but if anything it's probably easier there due to the higher amount of aoe healing going out at all times and the larger amount of available raid cds. You can even 3 heal or so 25H if everything is together.
I know how to handle it with 3 tanks and 3 healers. We've had several goes on them like that before we got roster issues. So I am not looking advice how to 3 tank it.
You need to time the debuffs and other things just when they first appear and then it is relatively simple... Even if we skip Shamans, I am still interested in 2-tank option for this, is anyone tried it and recorded it ?
Been 2 tanking this the last few resets, and it makes the fight a lot more complicated. I would absolutely not recommend it for a progression kill.