Everyone else is suggesting you buy a mixer, but if you are using a Devo 7, I don't think you need one. I'm using a Devo 7E myself, and after doing some research, it's perfectly possible to operate 2 wheel drive using one ESC. You just need to make a few modifications. Read up on these:

Rory's post alerting people to the Devo 7 series of transmitters, as well as info about the firmware needed to modify them with: http://robotwars101.org/forum/viewto...t=2387&p=56251

Userfriendly's guide on how to mix channels on the Devo, this should be what you're looking for: http://robotwars101.org/forum/viewtopic.php?f=10&t=2526

Hope this helps!