Now before you say anything, I know I'm insane...

[RC-Monster Mike]

Running two motor/controller systems off one set of batteries will work, but as most folks mentioned, it will be limited by the batteries ability to deliver the amps. I have done it before, but the runitmes were pretty short. I think the dual 8s motors carrying 8 cells each (each its own system) would be a better overall setup, to be honest with you. You won't have any trouble with power, either. One 8s with 8 good cells is fairly impressive in a 10 pound e-maxx, Two on your dagger will have some serious power. You could also consider LiPo power to reduce some weight. If you are thinking of dropping the $$ on a dual brushless setup, why not go right to the next level and jump to Lipos?