XY table using motorized potentiometers
For the H bridges, if the motor voltage is low i.e. less than 10V, please consider the implications of some types of H bridge listed in this answer. For the multiplexing of the potentiometers you can use a serial/SPI controlled analogue switch such as the ADG714 - it has 8 normally open switches but is restricted to logic level voltages.

Providing PWM control isn't needed (difficult to multiplex) the ADG714 can probably also help out - use the analogue switches to control enable and direction pins. Note that 2 ADG414s can be cascaded from one serial SPI bus so no extra pins are required.

If you are happy with constant speed I'd stick with it but you could implement a slow/fast speed regime by gating a 50:50 square wave oscillator to the FETs. You could take this further by using the LTC6992 PWM control chip - it needs an analogue input that could be provided from a serial DAC.