You answered your own question in the subject of your message.
You can use the -multimonitor command-line parameter for XWin.exe to create a window that spans multiple monitors.
However, this flag does not yet work in conjunction with the -multiwindow command-line parameter... though it would probably be trivial to make it work.
Are you using the -multiwindow command-line parameter?
No.
I have been playing using startxwin.bat.
If I add -display and -screen extensions I can create multiple screens and move them around.
If I use -multiwindow or -multimonitor nothing starts, is there anything to add to those to get a window to open?
I have seen some notes that they have not been documented yet.
-- Lester Caine ----------------------------- L.S.Caine Electronic Services
