What an awesome solution! am I correct in assuming that this was done mostly to make it look like one disk, and not to take advantage of the caching features of Fusion? (Since the disks will be about the same speed?)
I have almost the same setup, a 2 TB USB C drive using with my iMac internal SSD. The only reason I won’t Fusion them after reading this (great) idea is because I swap the 2TB between the iMac and a MacBook Pro to edit video on the go. But I have to struggle with symlinking Home directories (Download, Movies, and Pictures).
How cool would it be if the iMac just had a slot where you can slot in an SSD...