Why and when
In 2010 I got to ride a CB550 that had been modified with a second disc, and I decided it wouldn't be that hard to do to my CB550F. I didn't need more braking (or at least, not that attained by adding a disc) but I thought it was cool looking. So yes, this is a stupid mod -- it increases unsprung weight for no discernable benefit.
Below are pictures of the pieces needed to connect the speedo mechanism to the right side of the wheel, since some of existing pieces have to be scrapped to make way for the second disc.
modified disc with "D" drive piece "installed" (sitting) in the center
The pieces: modified disc (two slots added), "D" drive, "D" castellated cylinder
How to do it
castellation detail view of "D" castellated cylinder. "D" is cut to fit in "D" plate. Castellations are cut to match teeth of Honda speedo drive.
"D" detail view of "D" castellated cylinder. "D" is cut to fit in "D" plate. Castellations are cut to match teeth of Honda speedo drive.
detail of "D" plate. Ears must be cut to match slots you cut in the disc. The exact measurement is unimportant as long as it fits and isn't too sloppy.
Honda nicely has cast the mounting points for a disc on both fork sliders. Most of the work is just sourcing parts and bolting them on.
Remove the wheel and set it aside.
Mount the new caliper in the same way as the existing caliper.
Loosen and remove the 6 through bolts that hold the single disc on the wheel. Set the disc aside for now.
Remove "Cover, gear box retainer (44642-300-020)" and "retainer, gear box (44641-375-000)" and save them in case you decide to go back to a single disc someday.
Create the castellated cylinder by taking a piece of 1" PVC pipe and cutting slots in one end so that it matches the tits on the "box assy, speedometer gear" (44800-323-000). Diameter of 1" PVC pipe is close to that of the tits on the "box assy, speedometer gear". Length is about 14mm but you may have to make adjustments with a file on final assembly.
Prepare your second disc by machining two slots in it to keep the "D" plate locked to the disc rotation. Slot size is unimportant as long as they match the tabs on the "D" plate you make.
Using either plastic or aluminum about 1/8" thick, make the "D" plate. The ears should match the slots you machined in your new disc. The hole in center is the same diameter as the castellated cylinder you created above, but make sure to file a flat in the cylinder and to make a matching "D" hole in your "D" plate. I used PVC cut from a flat PVC 4" end cap.
Assemble wheel like so: from the left side of bike, the order is: old disc, wheel (remember to get rotation of tire correct), new disc. Heads of the new longer bolts go on the right side, nuts on the left side. The nuts are too tall to clear the speedo drive on the right side.
Slide the wheel between the fork legs, lining the discs up with both calipers. The "D" plate goes in the recess in the center of the disc, the castellated cylinder plugs into that, then the speedo drive plugs into the castellations on the cylinder. Now that whole stack can be slid between the fork legs and the axle installed. Make sure to get the speedo drive to align properly with the fork leg so the cable comes out approximately horizontally.
The assembled wheel installed in the bike.
How well does it work?
It still stops. You still have to fiddle with the ratio (master cylinder piston size vs piston sizes of wheel calipers) to get proper feel.