I created a mount for mounting a second motor on the X axis of an  Ender 3 or  Ender 3 Pro.
I used the original motor from the Y axis, which I replaced with the original extruder motor, which in tun I replaced with a pancake stepper.
This motor, originally on the Y axis in the back, has a fixed pulley 11mm away from the motor, so this mount is made specifically for that. You can use another motor with an adjustable pulley or you can modify the SW part file.
The mount includes a tensioner, through which 2 M3x35mm bolts go through to fix the motor.
Then, by adjusting the 2 M3x20mm screws on the side, the bolts holding the motor (and the motor itself) is pulled back to tension the belt. The bolts are attached to 2 M3 captured nuts.
The second motor is wired in parallel with the first X motor, using extension cables soldered to the existing cables and run through the back of the 2020 Al extrusion. You will need a cable and connector for that.
If you have installed my Dual Z mod, you will also need the updated X gantry…