We used to run Whirlpool with 4, 12v drills running at 15.6v on a single scorpion XL (with 2 small fans for cooling).

When we changed to 18v motors we added an extra Scorpion XL using one for each side.

Eventually we put a sidewinder in there (only because we are using the XLs in Heatwave and Whirlwind) and it has run without any problems at all.We got ours from http://www.robotmarketplace.com

Heres a quick video of Whirlpool when it ran with 4, 12v motors on a single Scorpion XL (it ran like this for almost 2 years without any problems)