The issue is that the added (2nd) caliper holder does not sit square to the disk (about 2mm out). Therefore the brake pads are at an angle to the disk and will not make proper contact. There are three solutions - 1) shim the upper bolt boss / machine the lower boss / machine the pivot pin on one side. I used the third option.
Just shim out the whole bracket with one or two washers between it and the fork leg at each hole... I've done this with success.
hold on ..... lets get back to basics ... the reason that the RHS caliper is skewed like it is in dbeerd's photo is that the adjuster screw is tighted up far too much on the RHS. Like K5 Keith says , the adjuster lock nut can be fitted on the inside of the caliper arm ... when stuff is squared up you'll find that there wont be enough thread protruding (if any) to put the RHS adjuster screw's locknut on the outside of the caliper arm.
The adjuster screws are only used to set the gap between the inner brake pad and the disc , that's all. ie JUST LEAVE THE ADJUSTER SCREWS & SPRINGS OFF UNTIL YOU HAVE THE WHEEL AN DISC IN PLACE Forget about looking to square everytning up before you put a wheel, disc and mudguard on .
If
not using a mudguard you will need to use shims between the fork leg and the caliper pivot mounting bracket ... if you
are using a mudguard you may not need shims .... BUT YOU WONT KNOW UNTIL YOU PUT THE WHEEL AND DISC ON and see how the caliper squares up the the disc..... that what you need to test at this point in time.
You still with us dBeerd ?