Most people tank swap on Trasher as the other tank goes up.
That being said I happened to tank 16-man HM Thrasher on Monday in crazy inter-guild group with at least one healer being really a sage DPS and I survived quite surprisingly even though we did it in tactics where guardian jumped up and I just stayed on boss at the gate. There were a few scary moments but I believe with healers knowing each other etc. it wouldn't be that hard. The only thing I did was respec for endurance talent for something like 38,5k health. So 16-man HM is not that much of a problem (although it still is much easier for a guardian or vanguard and much less dependent on carefully optimising gear).Based on my limited experience with 8-man NiM I can't see shadow tank in 16-man NiM for now. Maybe in full Kell Dragon gear from 8-man it would be possible though?
I did maintank thrasher 16 man hm for months (as an assassin) without any problems.
When you have a lot of random new people in your Group, it's way easier for them to just maintain tanking thrasher at a certain point while a juggernaut/vanguard jumps up with 2 dps or another assassin tanks aoe taunts the adds from downstairs.
First awall, these are not ideas... I do use phase walk excactly the way i described it. Kiting thrasher and tankswitch around is the main method for a good coordinated group in NiM Content (i think dilih and suckafish did it that way, just watch their vids).
Obviously its also easy to let the other tank tank thrasher at first for 5 seconds while you place your healing circle in the middle and take then thrasher for the whole time (and probably do a short switch when phase walk is back of cooldown).
What do you mean about HM Titan? Just place a healing circle in a position where the healers can heal the whole raid at 2 different launch positions. Thats quite easy and you get a lot of uptime (on the first two positions of Titan, partially on the third position and full uptime on forth position and burn Phase if you do it the optimal way).