I believe Andy Pugh on the LinuxCNC forum fitted 2 MPGs on the apron of his original lathe conversion but IIRC he says he never used them. As noted, you need a controller that can handle several encoder inputs AND the means to process them. The various Mesa Electronics cards that are used widely in LinuxCNC can provide that expansion but they would significantly increase the budget beyond the cost of a few Chinese MPGs, not to mention the trial of configuring them.

This may be like retaining handwheels on a conversion - seems like a good idea at the time but turns out not to be so valuable in practice - speaking as someone who retained handwheels on my first machine. A wireless MPG is soooo much better on various fronts.