Once upon a time, some smart people developed a way to "live boot" Linux from LiveCDs and other optical media. This was great, except eventually one developed a pile of obsolete discs. Then some smart people developed the LiveUSB. Which was also great, except one then acquired a buncha small-capacity thumbdrives each with its own distro.
Now we have relatively abundant/cheap large-capacity USB-connected drives. So wouldn't it be great if we could live-boot any of a number of ISOs on a single drive? Especially if one could
- relatively easily add/subtract ISOs as desired.
- boot from arbitrary hosts (as much as is feasible), rather than tailoring the LiveUSB to boot from a single/known host (as seems to be the case with, e.g., `update-grub`)
Hence the multi-ISO LiveUSB, or multiboot LiveUSB, was born. There are currently many tutorials on the web regarding how to make them: this tutorial
- is hosted on a public `git` repository, so it's easier to
- maintain/extend the information
- make pull requests (to help maintain/extend it)
- fork the repo (in case it's not maintained)
- explicitly discusses how to handle LMDE2 ISOs
- strives for completeness (hopefully less handwaving than some other tutorials)
Eventually the LMDE team can hopefully add material on this (IMHO) important topic to "the official documentation"; until then, hopefully you will find this tutorial helpful.