Brian;
For sake of discussion.

1 hp = 745.699872 watts
Let me see if I am getting this correct.
To get the best perfomance out of the 2 motors you would need to run different voltages.

The voltage for the 8xl should be in the 21 volt range and geared for a max of 74 amps. For roughyl 1300 watts.

For the 10XL you could run roughly 26 volts. There by decreasing the amp's need to get the same watts. Some where in the 60 amp range.

So if you had room to get 26volts of batteries in your car/truck you could would run cooler.

A few other things that being an electrican that I want to interject. As you increase your voltage your amps will go down at the same watt level.
Also the more the voltage the more efficent things become.
Example a 120v 100 watt light uses .83 amps
a 277v 100w light uses .37 amps The more amps you have the more heat you create (current), however voltage does not create any heat.

So if you were able to add a few more cells to the 10xl you would get longer runs time and stay cooler also it would be less stress on your esc.