We did it about a month ago with a warrior tank and a druid healer. It took around 56 seconds so I think the tanks cds lasted him through the added damage from Tenebron but since I wasnt tanking myself I am not sure. He did die in the very end before Sarth died (because the healer got so many adds on her) and after us having killed of Tenebron quickly the other two never landed for some odd reason. I have tanked it once since and chaining my cds when the first drake landed kept me alive til Sarth was dead even while tanking both of them.
If I remember the second time correctly one of the plate wearing dps chars used some AoE when the adds came to keep them off the healer and I think this helped the healer a lot from getting constantly pounded upon.
This was all done on 10 man btw as the previous poster asked about.