Allegedly, on or about 20 February 2019, Patrick Dupre sent:
I got a machine with 1 ssd and on HD. The ssd shows up in sdb, with the HD shows up in sda. I could not change from the BIOS the order.
For future reference: If they both use the same interface type, such as SATA, then you probably could have simply swapped the cables over.
(This may not help if what decides who's SDA or SDB is which drive gets ready quickest after power-on.)