You do not get double the torque with that setup.
If you’re familiar with planetary gearsets, you’ll know that they work by locking different components and powering different components for each ratio (some electric drills).
I unfortunately don’t have time to go into the physics right now, but feel free to message me and I’ll get back soon with a decent explanation.
But basically, the planet carrier and the ring gear in jpearman’s animation are BOTH powered by motors, so they only produce a ratio of 1:1 when they are unloaded, but as soon as you apply a load, one motor will encounter more load than the other motor, so you run into the case when one motor will actually fight the other motor, up to the point when one motor is completely stalled and the other motor is transferring its full torque to the output of the system.
Without locking some part of a transmission, it’s not possible to use a different combination of motor directions to change ratios. I ran into this when I made my differential-based transmission at the same competition as 4194B showed off their awesome drivetrain. You’ll notice (in the linked video) that I had to lock part of the mechanism when I switch to low gear, and in 4194B’s transmission that they had pneumatic locks when they switched ratios.