Check your screen's names by typing:
xrandr -q
Then disable one of them: (notice VGA-1 is the name of the screen, will differ, e.g. HDMI-1 if the screen is connected via HDMI)
xrandr --output VGA-1 --off
And then expand it:
xrandr --output VGA-1 --auto --left-of DVI-I-1
Depending on which screen turned black after the first command, you can choose left/right here.
At this point it should pretty much look like the final result, except you can't move windows to the other screen. Just right-click on the background and choose Logout... > Restart Icewm, be sure to click the arrow symbol for opening the log out menu.
And that did it for me.
I found a thinkwiki.org entry on xrandr which explains everything with nice examples, so you don't need to read manpages and figrue options.
Everything else is from a question in IceWM mailing list about dual screen.