Cracked it! Thanks everyone for your kind help.
My interface came from Russia today, I plugged it in (wow - ram upgrade! toolkit 2 onboard! hurrah) and immediately started on the floppy drive side of things.
I opened up one of the Sony drives, found the zero-ohm 'jumper' that was 'drive select 1', removed it and soldered-in a tiny
tiny jumper wire to bridge 'drive select 0'.
I tested this was now behaving as 'flp1_' by plugging it in to a really simple straight-through floppy cable. Yup, it was floppy one and if I powered everything down and replaced the drive with my otherwise identical other Sony drive, and powered back up, that one was appearing only as 'flp2_'
So then I took a long, standard PC 'twist' dual floppy cable, and carefully removed the IDC connector that was 'after the twist', undid the twist and replaced the IDC connector (a little way before the point at which is was connected before, to avoid the mangling of the original push-in connection). I trimmed the unwanted wire, connected it all up and ta-da! Done - I have a working flp1_ and flp2_, and I can happily copy between them.
Then I formatted a couple of true DSDD disks, then took them over to a PC that I set up with an external USB floppy drive and the wxqt2 tool. I got it configured (it does run in Windows 10 but you need to run the software as 'administrator') and I copied over a few files from the gms\QXL.win folder of my 'The Distribution' download, just making up a couple of real floppies with things like Unzip, breakout and a couple of other games.
I took these back to the QL and they worked just fine - I was even more surprised to see that disks in flp1_ that had a 'boot' program would auto-boot if I reset the QL - for some reason I just expected that to be MDV1_ only kind of thing.
Anyway, all ticking over really nicely now, so my next steps are to tidy this up a bit (wires and bare boards etc) then get a few bits of software to enjoy (probably an adventure game or two) and then - at last! - to start thinking about what I could write/fix/adapt/improve to help contribute to the community.
My final couple of questions for you dual-floppy owners;
a) how are you powering them? I've got an ATX pus from a PC, I've used a breakout board to trick it into powering up and I'm just running the floppies from 2 floppy-type molex connectors. I presume I could get a USB phone-charger type supply and just wire-up the 5v in to both floppies?
b) has anyone got a case or come up with a nice solution to enclosing the floppies? I remember the nice Cumana double-disk drive type enclosures, something neat to hide all that metal and ribbon cable would be nice. I may resort to some Lego Technics in the meantime.
c) has anyone come up with a nice way of covering/protecting the sticking-out section of the trump card? It looks like it's asking for a spillage or a short from a dropped pen etc....
Thanks again for the input. It went much easier than I was expecting it to!