If you connect two stepper motors in parallel as RAMPS does, each one gets half of the current. So you need to set the current to double in order to get the usual torque - but that may well be more than the driver can supply without overheating.
If you connect them in series instead as the Duet does for the Z motor connectors, both get the full current. But you need double the voltage available to achieve that current. Typical stepper motors used in 3D printers are rated at between 1.5A and 2A, and at low speeds drop about 3V. So even with 2 in series, the voltage drop at low speeds is only about 6V, and a 12V supply is plenty to drive them.