If you use one battery per motor (at 14.4v) then youll get quite good drive out of the motors, and your batteries will (should) last a 5min bout.
But if you use both batts on series (28.8v) then youll have halve the avalible current, and Id be surprised if you lasted even halve a bout. And there is another problem, the motors can only handle 14.4v 18v at extreme max, dont even think of using drill motors too far over their starting voltage. I tried a 14.4v drill motor at 24v and it simply disintergrated when I put a load in it. Stick with two batts in parralel...
Cheers, Ewan
P.S hopefully this helps...