I had duals on my 750 from 1972-1979. I used a second [normal] banjo for the second disc and made a bolt by drilling a 10mm longer metric bolt thru up to the head, then added an extra sealing washer between the banjos. I also had to shorten the bolt a bit as it ran out of threads up inside the fitting. I called it my 'power brake' setup, used in racing and heavy 2-up touring with camping gear for many tens of thousands of miles. The brake pad wear after 40k miles like this was tiny, and I used the remaining pads on the original one after I removed the 2nd disc in 1979 (needed some $$, bad, sold it to a neighbor with another 750) until 1988, then installed the other used pads until 2006. I'm only on my 3rd set of pads now, at 155k miles.
The main advantage I noticed with the duals was in city freeway traffic, and especially in the wet. Here in the Denver area it is VERY erratic traffic, and some of the steep hills here will hide dangers when you come over their top at speed. Then there's the mountains, which can make the front brake hot enough to melt fingerprints (don't ask...) after a few miles of spirited downhill...touring. While the dual disks required less grip force to slow the bike, I have not run out of braking on the single disk, not that I ever remember. The later cast-iron disks do grab harder with less braking lever force, but then I have not washed out the front end of this bike, ever, while I have come very close with some of the later, more modern ones on other bikes.
