you can overdub one or more tracks using an SD-Card and a USB-Stick at the same time, but the method is a bit clumsy - the complete procedure has been posted in the "old forum" some weeks ago, but it seems the old posts cannot be accessed anymore (?).
Anyway, in short, first you record the basic tracks to the SD-Card (must be class 10), using the option for recording single tracks AND stereo sum as well; then you play back the stereo sum only on the media player, routed via two monitor outputs into two inputs of your choice using patch cables, and record your overdub(s) to the usb-stick, including the stereo sum of your basic tracks as a reference for further editing in your DAW.
Obviously, the stagescape M20d, being a mixer, wasn't designed for overdub recording, but it can be done nonetheless;
if you have any questions concerning the details (eg, manipulating the input volume levels), please post again!