If you have connected both motors in parallel, then they share the current. So each one only produces around half the torque of a single motor. That may not be enough, bearing in mind the extra friction and inertia that the second motor + any associated drive mechanism brings. Increasing the current setting on the driver boards should help, if the driver boards can take it.
If you connect the motors in series, then at low speeds both motors work at full current and full torque. However, at high speeds there may not be sufficient voltage to drive both motors at full current. The solution is to increase the voltage to the stepper drivers. But with RAMPS, this will also increase the voltage to the hot end heater, even if you use a separate power supply for the heated bed..
If you connect the motors in series, then at low speeds both motors work at full current and full torque. However, at high speeds there may not be sufficient voltage to drive both motors at full current. The solution is to increase the voltage to the stepper drivers. But with RAMPS, this will also increase the voltage to the hot end heater, even if you use a separate power supply for the heated bed..