@axiom If you use two motors on one driver (either by using the two outputs on the Z axis, or by wiring the motors together and connecting them to the single output on any driver), that driver will have to work a bit harder, but so long as these aren't particularly big motors, or trying to go very fast, it should be fine. Ideally you'll be using a 24V power supply.
With the motors on separate drivers, you are spreading the load, and the motors can even be controlled independently. That's why people often use them for the Z axis, because the axis can then be levelled automatically. People with CNC routers often have two motors on Y, and put these on separate drivers, which allows them to square the X gantry to the Y axis. Normal commands to the axis are carried out by both motors unison. Though this isn't a particular advantage in the way you want to use them, as your motors are connected to the same belt.
I'm assuming you're using RepRapFirmware 3.x. If you are older firmware, please post your current config.g and the response to M116.
X motors connected to different drivers
If you connect the second motor to E1 (driver 4), you should add an M569 to set that motor driver. So add something like this to config.g:
M569 P4 S1 ; physical drive 4 goes forwards
M584 X0:4 Y1 Z2 E3 ; set drive mapping
If the extra X motor turns the wrong way, change M569 P4 S1 to M569 P4 S0. There's also an example on this page https://docs.duet3d.com/User_manual/Connecting_hardware/Motors_configuring#using-more-than-one-motor-on-an-axis-with-a-separate-driver-for-each-motor
X motors connected to Z driver
If you connect both X motors to the two Z driver outputs (the labels don't really matter, the Z driver is driver 2), it's probably easiest to swap the Z motor onto the X driver (driver 0), assuming you only have one Z motor. This leaves your E1 driver still available:
M584 X2 Y1 Z0 E3 ; set drive mapping
If the extra X motor turns the wrong way, you'll need to adjust the wiring of the motor, and swap the two wires of one of the phases in the plug. See https://docs.duet3d.com/en/User_manual/Connecting_hardware/Motors_connecting#using-the-internal-drivers for an explanation of the motor phases. Lower on that page is a description of connecting two motors to the ZA and ZB connector.
X motors wired together in series, connected to the X driver
Alternatively, you can wire the motors themselves in series to one plug, and connect it to the X driver. You won't have to change M584 for this:
4072e6c5-0875-4a14-aa76-15aefae64f8c-image.png
If you're still not sure, please post your current config.g and the response to M116, so I can see what firmware you are on.
Ian