everything seems to assume that the two OSs are on the same physical drive
No, not really. It's up to you to choose the location when installing the OS.
Simply causing the bios to read the chosen disc is what I'm aiming for
You will live to be disappointed - the BIOS is nowt to do with selecting the OS.
Get both disks recognised in the BIOS, then leave it alone.
The system boots off the MBR (Master Boot Record). This is the first sector of the first hard drive (called the Master or HD0). This then points to the boot files on the active partition and starts the load of the OS.
In a dual-boot configuration, there is an extra step before the OS loads. The system still boots off the MBR, but before the OS is installed, the bootloader or boot manager is read, and this allows a choice of OS to be installed. Note that the boot manager is part of the (usually most recent) OS that was installed.
If you simply install an OS on disk 1, then do a standalone install of another OS on disk 2, having replaced disk 1, this will not work as a dual-boot configuration.
Why not read up about the
boot process
SD