Using two motors instead of one could have a benefit if each motor powered a different wheel because that would create the equivalent of a limited slip differential which would help with traction. Other than traction there would be no advantage to using two motors instead of one. If the vehicle is being used on mostly flat sidewalks traction will probably not be an issue though. One controller can run two motors and both motors will run at the same speed as long as they are identical motors. Two 36V 250W motors could be run off of one 36V 500W controller.
Please let me know if you have any further questions.