How to make two internal SSD drives work best on a Mac
I haven't tried this myself but you should be able to create an APFS volume group from your two drives to effectively create one &quot;Fusion&quot; drive. You would use
sudo diskutil apfs create

or
sudo diskutil apfs createContainer

to accomplish this. You can run those commands as-is to read usage. Also see the manual page for diskutil (man diskutil).
The advantage of a Fusion drive is that you don't have to manually manage which files get put onto which drive; the whole thing behaves like a RAID 0 array (i.e., JBOD, &quot;Just a Bunch of Disks&quot;). The downside is that you're now multiplying your chances of total data loss if there's corruption on either of the drives. So before beginning, you are very strongly advised to create AND maintain a backup of your data.