I've heard of this both being an arbitrary BIOS limitation (in which case Linux with kernel command line to override the BIOS to specify the floppy drives would be a good test) as well as the Super I/O having multiplexed pins, such that dual floppy has to be sacrificed if the board maker wants some other feature (like parallel port).

I have run into a few systems which only supported one floppy drive in hardware, the select for the second drive was not brought out (perhaps saving a pin "that nobody ever uses" on an IC)

Assuming you have a proper "twisted" cable supporting A: and B: - Boot DOS on the working drive, and try accessing the second with ImageDisk.

IMD does not use BIOS - accesses the floppy hardware directly (I have both drives set to "NONE" on my IMD system - to avoid anything else trying to "mess" with the floppies).

This will tell you if the second floppy hardware is connected and working.

Also, since it doesn't use BIOS - IMD can easily use differnt drives connected though the same floppy interface as well. I do this to support many different drive types directly (5.25" - 360k 1.2m, 3.5" - 720k 1.44m, 8") - I just have a "power switch box" which cuts all power lines to a drive while I change it.

but ... for "normal" use, BIOS has to be configured for the correct drive type. There may be a tool somewhere to "restore" BIOS on the fly - but I've never really looked .