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.
Yes, it would be handy if the taskbar could be duplicated (or triplicated, in my case) among all available screens.
What I find disturbing is that, using the same exact configuration, whenever I reboot my machine and connect the screens, the IceWM taskbar goes to some random (apparently) screen, it is not predictable.
I'd like to send it either to all screens, or to a particular one, but so far I haven't found a way to do it.
Has anyone some idea, to address this problem?