There's a few issues here. One is picking a rotor dia that has cost effective choices available. 310/320 mm rotors are by far the most common for road bikes. Another is as OD increases so does the speed/friction/heat differences between inner and outer rotor section. This is why they use small mulitple piston calipers on narrow rotors now instead of one big ole pad like Lockheed's, or else get wrapped rotor issue. So if you pick the most common rotor size then you shoot self in break pad foot of Norton's art deco sculptured caliper I like so much too.

The adapter plates look rather inelegant stop gap mounts with extra parts and mass to me. I fitted rare 290 mm wave rotor to Ms Peel w/o adapter gizmo but does leave ~1/4" of Lockheed pad over hanging the rotor rim. I figure the extra OD leverage and rotors friction increasing edges and lighter spinning mass to slow will more than make up the slight lost of total swept area. Not sure what to do the the excess pad edge, just leave it alone for bit more heat radiation area and let rest of pad wear a lip or grind off to match rotor arc. Trick was to not foul the floating fasteners. I assembled everything with brake lever squeezed then welded the piece in there so assured aligned and clearance once cooled.

RGM sells a dual disk hub reasonably priced. I am about to order one this week. I have one disk now, but a future project is to add the second, and a slider with the mount would be great.

Hobot what is your rotor thickness ?

Its a race only 290 mm - 11.6" BraKing wave rotor, 1/4" or 5 mm thick, model STX400 4mm minimum thickness, floating button mounts. Found on ebay cheap as full retail is couple $100+ at least.

I practice braking as most important scary and hardest skill to learn. I'm no stunter yet but have stoppied a SV650 and Ninja but they do not stop as short and secure as my single brake Commando's. Ease of lever is one thing but the real issue is tire grip and controlling that w/o lifting the rear much or means head over tea kettle unless backing off braking effort. With this experience in mind I figured less spinning and un sprung mass was better way to slow and stop my Cdo's. I actually expect to beat elite sports bikes with ABS in 100 > 0 time and distance. They may have a balloon tire which looks the business but hehe they still have similar tire patch area so only tire compound and air pressure matters in front braking grip. Its dangerous to panic grab brake near 100 on the moderns w/o ABS but not my Cdo's which just squeal loudly w/o ABS jerks till under 60 then must back off or lock up. You can't really stop where intended on THE Gravel with ABS. Dual discs have more heat tolerance before fade by sharing heat loads so may work better for tight tract racing conditions.

Window shop here
http://www.kneedraggers.com/list/1226/bike/Suzuki/SV650