I'd call it akin to an open differential.
Regardless, if you want the motors to run slower but both always get the same voltage (which is the closest thing to something like a posi-trac), you will have to keep them in parallel and either retrofit lower-speed motors OR run a lower voltage to them. You could use a PWM controller or a strong buck converter to reduce the effective average voltage. However, doing this reduces the maximum amount of torque you can get and if your goal is for it to be self-propelled across sand or other high-drag surfaces this may not be that great.
If you went to a high-turn motor with a 8-10,000 RPM speed @ 12V, you'd get 2.5-3mph and likely retain more of the torque. For instance, this one: https://www.robotshop.com/en/rs-775-mot ... oz-in.html
The stall torque is a little over 2x that of the stock motor, and the speed is a little less than half. I don't know what size wheels you're using, but it would do ~3mph with a 22T gear (in a 21-22-23 gearbox) and 15.5" tires, and it would have a bit more torque than the stock Hurricane (15-tooth) gearboxes do.
There are better choices (I'd target 10-12k rpm), so that's just one example.
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (no, it's NOT all-wheel-drive--look HERE
for more details!)
Pink Rocket Princess Mustang - 15mph, 36V, 4x 775 motors, homebrew PWM controller rev 2....
Hurricane - 24V w/50A step-down controller, 775 motors