Size is an issue, a comparable industrial motor with the power I need (150-200kW continuous) and the speed I need (about 500-600rpm) is very big and heavy.Do you specifically need the Flender? There are other choices...
Ready to go: new summation gearbox for electric buses and mobile machines
That one is new so you won't find a salvage/surplus unit, but it seems likely that there are other brands.
The whole idea of a summation gearbox seems strange to me. It just allows the use of two smaller motors rather than one larger one... but electric motors scale up very well, so one larger motor is cheaper and more efficient than two smaller ones, and avoids the need for two inverters. I suppose it's just the easiest way to use available surplus motors that are not the right size.
That's the production Two-Mode hybrid system, used in GM light trucks (full-size pickups and SUVs) for a few years, and available from Allison as the H 40/50 EPTM series, used mostly in buses.Only thing better would be a gearbox where the motors are not in the line of the output shaft, so I could combine a diesel engine with two electric motors
(think boat, parallel hybrid / serial hybrid)