voltage dictates motor spin speed if everything else is equal, so unless you've got a method to make the controllers keep them at the same speed - with those packs, one wheel will try to run faster than the other i would expect. Not sure how that would handle - or if you'd get any increase in range.
The slower motor might even brake the faster (or vice versa - the slower motor is spun faster than it's powered speed) - so causing it to heat up but I'm really not sure of the actual result of running two motors at different speeds without clutches or similar - probably depends on a number of factors.
Swapping packs once one is discharged and powering just one wheel from a single pack would be the simplest way to increase range - but you'd need a switch or other method to change packs.
