Setting up SSD with Catalina install
I think I have what I needed (a combined drive) all of it available for use. It is now listed as a Fusion drive even though both its components are SSDs. Here are the steps I used to combine the 2 drives.

If you're using macOS Mojave or later

- Turn on your Mac, then immediately press and hold Command-R to start
   up from macOS Recovery. Release the keys when you see the Apple logo
   or spinning globe. When you see the macOS Utilities window, choose
   Utilities &gt; Terminal from the menu bar.
 - Type diskutil resetFusion in the Terminal window, then press Return.
 - Type Yes (with a capital Y) when prompted, then press Return.
 - When Terminal indicates that the operation was successful, quit
   Terminal to return to the macOS Utilities window.
 - Choose Reinstall macOS, then follow the onscreen instructions to
   reinstall the Mac operating system. Your Mac restarts from your
   Fusion Drive when done.