I really like the idea behind this - I think it may have applications beyond just drive trains.
Something you may want to look into is gearing the upper and lower motors at different speeds, with a non-backdrivable reduction for the high speed motor. This would let you get a wider range of gear ratios without having to run motors at really high speeds.
This paper, if it is ever released, describes the exact same idea in a planetary differential with some equations/control ideas that might be useful.
https://ras.papercept.net/conference...=103&Number=70