Figured it out after reading the Unity manual.
Make a shortcut with these parameters:
Stranded_Deep.exe -force-d3d11 -screen-fullscreen 0 -popupwindow -screen-width 3840 -screen-height 1200
Replace 3840 and 1200 with your combined screen area. Works fine with different monitors, too: one of mine is only 1080p so a bottom bit of P2’s screen is not visible, which is a better deal than having the larger monitor cut down to 1080 as Eyefinity does.