i only have anushri so far but am planning my rack similar to yours so had to try! I was of the impression that most euro modules and patch cable are mono, i.e. that is the norm, although there obviously some that do stereo, sometimes this would be on one minijack (tip/ring/sleeve) but more likely be separate monos for l+r. These are usually fx machine and have some kind of panning or width/spread adjustment.
what you need for head phones is mono to stereo converter jack, these come in 3.5>3.5 or 3.5>1/4