If you need a two-needle instrument but don't have a suitable mechanism for one stepper or multi-shaft steppers, you can use separate steppers for each needle with additional gear wheels.
Both gear wheels have to be of equal diameter:
The second (bottom) stepper can have a zero-position sensor disc directly on its shaft.
One stepper motor should be configured with inverted mapping. To avoid this, you can add a 3rd gear wheel as shown in the picture below. Or, any other gear combinations can be used, depending on your gauge design (for example, when you have two co-axial needles with gears as in the right picture: