As long as you stay within the max current limits of their respective drivers, you can run two motors on each axis. The RAMPS really is not the determining factor, except that it allows to easily connect two motors in parallel for the Z-axis, by having the extra connector already provided for. The determining factor is the stepper driver, and as evidenced by the numerous Prusa i3's that are printing without any problem around the world, two motors is OK with either the A4988 or the DRV8825 stepper ICs.
