What version of nitrogen are you using? My daughter is on her laptop so I can't access the VSIDO computer right now.
In Void, the version is 1.6.1 and I have this option:
--head=[arg]
Select xinerama/multihead display in GUI, 0..n, -1 for full
try "nitrogen -h" in a terminal to see what your options are.
I also have an extra button between the "Zoomed Fill" and "Color" button that looks to be a gui option for selecting the monitor to use. I don't have a dual monitor set up to verify so strictly theory here, no practical application.
