TOPIC:

I noticed that the 750 has room for a second caliper set up. How would I go about adding a second rotor to the stock spoked wheel? I am used to mags that have the threaded bolt holes for a rotor and am not familiar with how to add it to a spoked wheel.

theres a cover stamped out of sheetmetal hel on with 2 screws on the hub,remove cover and you will see 4 bolt holes like the other side to mount rotor to.

I think wireman got that right on the money, heres what it looks like on a 650, should be the same thing.

Wireman described it perfectly
If you look at my avatar you will see an 80 KZ750G twin with dual disks, I pulled the parts out of my large supply of KZ parts, so I didn't have to buy anything.

I suggest using the rotor off a dual disk 650 or 1000 as they are considerably lighter than the single disk rotors, and if you have the stock master cylinder (on a 78-79 twin) then it is already 5/8" and you can use it.
If your caliper mounts in front of the fork, you can swap the fork legs side to side to mount the calipers behind the legs, but your fender stay mounting will be on the wrong side so you will need to remove the fender stay.
A dual disk brake line splitter would be nice, but it's not required.

79 KZ650SR (don't know what others) has the lightweight rotors and they are drilled stock.

So all I should need is another rotor mount and hardware to get a second rotor... Thanks guys!