I use dual monitors all the time in Lightroom, keeping the photo I am working on full size on one (calibrated) monitor, and the LR control panel on the other. However, when in LR I desire to edit in Photoshop, I previously thought (when in PS) that I no longer had this dual monitor editing option. But then I experimented, prompted by this thread, and discovered that when in PS I could go to the "Window" tab, then the "Arrange" sub-tab, then choose the Float in Window option. I could then move the photo I was editing in PS over to my second preferred monitor, while retaining the PS controls in the first monitor. All very easy and striaght-forward (at least for me).