Since nobody better qualified has answered, I'll take a crack at this one.
Simply looking at the current draw, it looks OK. But it's close enough that you might want to verify the current draw just in case the motor makers took a little "creative license" with the specs. Check the current under both no load and max load.
But, what I would be most concerned with is the speed of the motors. It's almost certain that two motors will not be matched in speed. And even if they are, they won't stay that way due to age, wear, and differences in weight distribution and terrain.
This difference in speed, especially with a geared motor that won't tend to slip, will manifest itself as stearing problems, increased wear on the wheels, and other bad things.
But, assuming that the current spec is correct, I don't see any harm in just trying it. There is a chance that I'm wrong and they will be matched enough that it'll work just fine.
You've got the right idea with PWM, but relays are totally inappropriate as the switches in this case. You want to run the PWM at least a fw 100 Hz, and that would be slow. That's fast enough for the motor to work well enough, but could cause audible whine. That is why most motor controllers pulse at 25-30 kHz, just past the audio range.
Probably the best way to switch the voltage to your motor is with a low side FET, or perhap several in parallel. Use good FET gate drivers intended for that purpose, and control those from the PWM output of your microcontroller.
A starter motor is not really a good fit for what you are trying to do, but you should be able to get some results like you expect. In general, start motors take a lot of current, have high torque, but aren't meant for sustained operation in several ways.
Best Answer
You can do it yourself or buy it. It's your decision. We don't know how experienced you are, so nobody will tell you what to do.
If you have no experience with any PWM motor drivers - you should mess around with low power motors first.
PWM regulator for 1000W/48V motor needs good overload protection, because it has to be powered from very low impedance source (big battery with short-circuit current probably over 100A).
In addition - regulator for vehicle must be able to handle voltage (or power) coming from motors while slowing down, because motors work as generators.
Edit:
I found something useful for you: How can I implement regenerative braking of a DC motor?