Silver,
I definitely understand where you are coming from, I would have let Tank 2 die. From what you said it was clear he was trying to get aggro mid fight for the hell of it. I would have let him die, discussed this with him before rezzing, and when he said, who cares, kicked him from group and let him fend for himself to get his shard. But thats just me.
This is also why I avoid multiple tank groups, as I have had 2 tanks battle for aggro before as well. Not fun, no reason for it, more down time. End result stick with 1 tank groups when you can.