Are there commercially available reduction gearbox units that have 2 inputs shafts. One input shaft for a high speed motor and one for a low speed motor.
You can get double extended input shafts on gear reducers from a number of manufacturers. You will increase the drag on the operating motor if you keep both motors coupled to the gearbox. If the only reason you are doing this is to get two speeds you should think about two-speed motors. A variable frequency drive for a single motor will give you a wide range of speeds, easily 6:1 turndown ratio. Depending on the horsepower that you are transmitting this may well be cheaper than buying two motors, couplings and drive bases.
blob,
dvd is assuming your are referencing electric motors. If you are refering to hydraulic motors, you will still have a drag problem (very small) from the idle motor, but you can get hydraulic pump drives from a number of sources (Durst div. of Regal-Beloit, FUNK, EATON...etc.) that will work equally well in reverse (i.e. a hydraulic motor to drive the shaft.
