@simonsays If you're new to Linux, the easiest thing to do is have one SD card for Debian, and one SD card for Recalbox, and reboot with the other card to switch between them.
EmulationStation does not support running at the same time as the desktop. It is possible to install both on the same card, and type "startx" at bootup to run the desktop or "emulationstation" at startup to run recalbox. But the actual installation is very advanced, and it complicates getting help with questions. So as I said, the easiest approach is just to have two separate SD cards.