The problem with connecting two motors in series is that you need a speed controller that will handle 24v+ (to allow for fully-charged cells) and that one motor/running gear combination will always be slightly more efficient than the other. This will result in one motor slowing down and the other speeding up. I would advise against it. There is nearly always space in a tug for a smaller 12v battery to augment the main 12v supply; smokers don't take even an amp so a huge battery isn't necessary. Personally I'd buy a 12v smoker to start with, or fit a DC/DC converter if I already had the 24v type.
One last throw of the dice; the ACTion P93 Multicontroller will handle 24V (and 10A) and has a 50% ESC output mode. This will effectively supply 12v to the motor. More details here (click on P93 Info button) **LINK**
DM