not sure i did a step by step, but it is very easy...
yam wheel has a cover on opp side of hub and pry this off to reveal the machined holes for another disk.
1) fit another disk
2) you need a left hand early type lower fork part. they have a fitting for another caliper mount
3) use a caliper and mount from an SR500 . these are the same as xs400 right side, but it says left or "L" on it
4) buy 1 longer brake pipe and a double banjo fitting for the master cylinder
5) use master cylinder from an xs750, 850 or XS1100 twin disk bike. I would rebuild it with new seals
6) bleed system for 2 hours and there you have a twin disk set-up
drewps