Using two feeders at the same time
The first multi material add-on version off J. Prusa includes several separate steppers that load filament into the hot end. Note that selecting an other color will retract the existing filament so that the next filament can be loaded. There is no mixing hotend nozzle. It is therefore working serial, not parallel. Working 2 steppers at the same time will  lead to clogging the filament joining position.