Hi Jon
Yes you can use two ESCs to control a tank
For 1 stick control:
You just need to program into your radio what the flyboys call a V-Tail mix
OR
2 Stick control:
Program left esc as throttle on left stick and right esc as trottle on right stick
Doing this would not cost you anything but you would not have ANY turret functions unless you knew a bit of electronics and had a few servos lying around.
Your best bet would be to find someone who has upgraded to a El Mod system or the like and are willing to part with their old HL electrics. Someone like Reaper for example
Cheers
Kevin G