Why does my new bootable external SSD show up as 2 volumes?
You are seeing Catalina's way of setting up boot volumes, that's all. In Catalina, when you install macOS on a drive, it gets split into two volumes behind the scene: one read-only where the system and all system files reside, and one writable where your user files and every file that must be writable resides.

This is a way to further harden and protect the system from any accidental or malevolent change. The split is done by some file system level 'magic' and the volumes occupy overlapping space on the physical disk.

The preceding is a bit simplified, but you get the idea. When you see this volume in Catalina, it is presented as a single volume to the user. But when you see it in Mojave, it is shown as the two volumes it appears to be for Mojave, since Mojave has no knowledge of Catalina's boot disk setup scheme.

To use under Mojave, you can just ignore the volume without the "- data" and use the volume with "- data". All changes you do to that volume (as you have already seen with the icon) should show up back in Catalina.