If a spell must be cast with both hands, it will select itself with the LR> mark when you activate it -- If it only selects with L> or R> it is cast with one hand. One hand spells can of course be selected on both hands, but trying to dual-cast that spell (without the 'empowered' perk, or if that spell cannot be empowered) you will simply attempt to cast the spell once with each hand.

In regards to the actual ability, I haven't gotten a chance to use this mod recently, so I don't know exactly how he has that set up.

I also seem to have a trophy bug too.
When I gave the soul of Alduin to the lady there, Alduin's trophy shows his body having skin, but his wings are only bone. you can see the skin that should be there, but it hangs like strings on an instrument.
Any way to fix this?