You can kill someone who's at two life with Twincast, however.
I don't see how thats possible. On each resolution of the spell, there life total goes down and comes back up within one spell/copy resolution. Gamestate is not checked in between looseing life and gaining life, so there is no point in time where gamestate is checked, and your opponent is at 0 life. In order for you have an opponent's tendrils actually win you the game, you need to have sort of life gain replacement effect (sulfuric vortex, false cure, etc).
Also to really rules lawyer this one:
If your opponent is at 2 or less life then tendrils you, you can misdirect the original, the copies stay on the stack, to him. He would loose 2 life first before the copies resolve
You would actually want to misdirrect the first COPY to resolve. So let the storm trigger 'unpack' (resolve) and then state that you "are casting misdirrection targeting the first copy to resolve from the stack." If you target the actual original card, then it will resolve AFTER all the copies do. Take this example:
Opponent is at 2 life, you control Sulfuric Vortex (whenever a player gains life, that player gains no life instead)
Stack top:
Storm Trigger
Tendrils of Agony (the card)
If you cast your misdirrect now, when the stack finally resolves your going to have:
MisD
Storm Trigger
ToA the card (targeting you)
after MD Resolves you have...
Storm Trigger
ToA the card (targeting your opponent)
then the storm trigger resolves...
Copy of ToA Targeting you
Copy of ToA Targeting you
Copy of ToA Targeting you
...
Copy of ToA Targeting you
Tendrils the card Targeting your opponent.
So you are going to get stormed (to death presumably) before your misdirrection target actually resolves.
NOW--
If you wait until the storm trigger resolves then target the first copy instead you have...
Copy of ToA Targeting Your Opponent
Copy of ToA Targeting you
Copy of ToA Targeting you
...
Copy of ToA Targeting you
Tendrils the card Targeting you
And when the Copy resolves, your opponent goes to 0 life, and the vortex replaces the life gain ... and you win!!