I would say 750 as well.

The higher LVC won't keep the batteries balanced to each other, it will just help prevent problems that could arise from them not being matched together.

Oh ok. I was reading about the ESC and it doesn't have brakes so I would have to run mech brakes which I don't think is possible with a 2 battery set up. I guess I'll have to wait for Lutach to release his 12S ESC.