I got this working with code like this:
XineramaSlsSize 2x1
XineramaSls On
XineramaPrimaryScreen 0
Xinerama On
*FvwmIconMan: NumManagers 2
*FvwmIconMan: Resolution screen
*FvwmIconMan: ButtonGeometry 1100x0
*FvwmIconMan: 1 ManagerGeometry 1x1+0-0@0
*FvwmIconMan: 2 ManagerGeometry 1x1+0-0@1
One confusing issue was that the screen name starts at 0, but the icon manager name starts at 1.