Well, it is a bit more complicated and, surprisingly, you both, @Mr_L_on_Yoshi and @tmwilliamlin168, are correct to some degree.
Lets look at this analogy:
If you assume that person on the left could provide a unit of lifting force and a person on the right could provide twice as much, then together they could lift a weight corresponding to the two units of force. Left person will struggle at 100% of his/her max, while the person on the right will be underutilized at 50%.
Now, imagine you add some extra weight. The person of the left couldn’t handle this anymore and will start dropping the sofa, while the person on the right will keep lifting it up. If you assume that left side drops as fast as the right side is being lifted, then the center of gravity (of the sofa) will stay in the same spot. In case of differential, when the resistance at the output is too high for one of the inputs to handle, the single motor side will be backdriven by the side with two motors, .
So far we assumed that max torque out of the motor is a fixed number. However, electric motors deliver almost no toque at the idle speed (just enough to compensate for internal friction) and, as you start adding load to their output, they will be slowing down to generate additional torque:
https://vexforum.com/index.php/attachment/56464d7f91a13_torque_speed_393.jpg
(I assume everybody has motor-torque-speed-curves diagram in the eng. notebook)
Lets look at our unlocked differential. As you increase the load on its output, the side with one motor will slow down twice as fast as the side with the two motors, such that torque coming from left side is equal to the torque coming from the right side. At some point single motor will overheat its PTC, “drop the sofa” and will be backdriven by the other side, which still have plenty of power margin.
I don’t know what @Mr_L_on_Yoshi has in mind, and assume it is not a simple unlocked asymmetric differential which would be a waste of a motor.
If there is some sort of a locking mechanism, such as brake, ratchet, or worm gear(s) then you could create very interesting system by preventing the backdriving of the weaker side. You just have to be careful not to make it too complex, because then it may introduce extra friction and reduce overall reliability.