Motor assisted pedal powered rail rider

I am trying to build a 2 person human powered Vehicle that is also motor assisted, I need a way 2 people can pedal at diffrent speeds and drive the same axle. What I think I need is some sort of differential gearbox. What Ii need to know is:

Is there is a differential that can take 2 different input speeds and have one output?

If it did and I also pushed it with a motor would the wheels spinning feed back though the gearbox and send the pedals spinning like crazy?

Any help or advice people can offer would be great.

Won't really work like that.
But you can substitude the hubs of two mountain bikes together with pedals and chain.
That way all riders stay independly although one can pretent to paddle and just be lazy ;)