Thanks very much to all who provided helpful suggestions!
I finally (with help) got it all figured out...
1. Dual Disk Hub options:
https://risseracing.com/store/index.php ... 0e2d5b00aa
I ended up ordering from the last one.
2. Mirror Image calipers:
In the end, I found matching L&R 8 piston Gator calipers and mounting brackets, on eBay in Germany (4 and 6 piston were also made in RH and LH versions):
https://www.ebay.de/sch/i.html?_osacat= ... e&_sacat=0
NOTE Gatorbrakes are no longer in production, so pick up some extra brake pads on Bay if you go this route.
'Rechts' is 'RH Side' in German, and 'Links' is 'LH side'. 'Vorne' is 'front' and 'hinten' is 'rear'. Just copy and paste text into Google Translate.
I will post again once I figure out the master cylinder requirements, which I will do using proper industry standard hydraulic engineering protools (piston size is critical, relative to piston size in the calipers).
Also still needing to sort out are a hydraulic splitter, or banjo bolt arrangement to allow two hoses to connect to the master cylinder...