The DC thing kinda explain it
The logic is simple, for instance having a plate DPS tanking Garajal instead of an actual tank spreading the stupid excessive damage he will be taking with vodoo doll to other players makes this alot harder tham 2 tanking it. Pretty much everytime youre using a DPS to get smacked in the face instead of a tank youre making things less stable cause there will be atempts were your DPS will get critted twice in a row and die in a split second. 1 tanking Will makes stuf alot more complicated for your melee DPS for instance and put excessive strain on one single tank, wipes are bound to happen cause of that. If 1 tanking all those fights were somethign stable the tactic would be alot more popular and it isnt because ppl kinda have to go out of theyre way to do it, in the Garajal case for example you cna egt extra DPS by using 2 heals and keeping both your tanks.
Most likely whats happening here is your guild has some awesome healers and youre probably quite above average as a tank so you guys manage to pull off some of these pretty unorthodox strategys but dont say these should eb the standard cause they realy shouldnt and 99% of every guild will 2 tank everything next patch.