Creating a "virtual" dir that wraps two (external) drives

I was wondering if MacOS Mojave (the most recent version I can be on due to some very expensive 32 bit applications) has a way to set up a system level "virtual" dir, or virtual drive, that combines two or more external drives and surfaces them as if they're one directory/drive to all applications. E.g. something that lets me say "create a virtual dir/drive called audio data" (that *isn't* JBOD/RAID0) and then specify two (or more) external SSDs such that the content of that dir/drive is the combined content of the solid state drives. If one has dirs Pro Logic data and Cubase data and the other has dirs Kontakt and Samples, then I'd like a single virtual dir/drive that shows:

--- data
 |--- Cubase Data
 |--- Kontakt
 |--- Pro Logic Data
 |--- Samples

When both SSD are connected, or if one of them's not connected, show

--- data
 |--- Cubase Data
 |--- Pro Logic Data

or

--- data
 |--- Kontakt
 |--- Samples

depending on which drive's still accessible. Or a flatout empty dir/drive if nothing's connected. With the explicit intent that if the SSDs that house dirs that some applications rely on aren't connected, then yes: those applications will throw errors. That's fine, and expected. I just want something that lets me surface a ton of data spread over multiple drives as a single dir/drive when they're all connected, with each SSD being "just a drive" that can also be connected to any random other computer that knows how to read/write the SSD's filesystem.