Stretch window over two monitors with "gap" in between

I have a dual monitor setup with Compiz as window manager on Ubuntu 14.04. I can resize a window to stretch over both windows. However, while the physical display panels are a few centimeters apart, the software by default thinks of them to be right next to each other without a gap in between. This means, e.g. a diagonal line ends up looking like this:

+----------+ +----------+
|          | |   /      |
|          | |  /       |
|          | | /        |
|          | |/         |
|         /| |          |
|        / | |          |
+----------+ +----------+

What I'd like to achieve is something more like this:

+----------+ +----------+
|          | | /        |
|          | |/         |
|          | |          |
|          | |          |
|         /| |          |
|        / | |          |
+----------+ +----------+

This would result in part of the window being hidden, but that's acceptable.

I can make X regard the displays to have a gap in between as follows:

$ xrandr
Screen 0: minimum 320 x 200, current 2048 x 1280, maximum 32767 x 32767
LVDS1 connected (normal left inverted right x axis y axis)
   1366x768       60.0 +
   1360x768       59.8     60.0  
   1024x768       60.0  
   800x600        60.3     56.2  
   640x480        59.9  
VGA1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 disconnected (normal left inverted right x axis y axis)
HDMI2 connected primary 1024x1280+0+0 left (normal left inverted right x axis y axis) 376mm x 301mm
   1280x1024      60.0*+   75.0  
   1280x960       60.0  
   1152x864       75.0  
   1024x768       75.1     70.1     60.0  
   832x624        74.6  
   800x600        72.2     75.0     60.3     56.2  
   640x480        75.0     72.8     66.7     60.0  
   720x400        70.1  
HDMI3 connected 1024x1280+1024+0 right (normal left inverted right x axis y axis) 376mm x 301mm
   1280x1024      60.0*+   75.0  
   1280x960       60.0  
   1152x864       75.0  
   1024x768       75.1     70.1     60.0  
   832x624        74.6  
   800x600        72.2     75.0     60.3     56.2  
   640x480        75.0     72.8     66.7     60.0  
   720x400        70.1  
DP2 disconnected (normal left inverted right x axis y axis)
DP3 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)
$ xrandr --fb 2184x1280
$ xrandr --output HDMI3 --pos 1160x0

At first sight, this seems to work perfectly – when I move the mouse in a diagonal line across the gap between the monitors, it disappears "behind" the monitor bezels and re-appears where I'd expect the diagonal line to continue.

However, I can no longer move or resize a window to stretch across both monitors. Neither the mouse nor wmctrl seem to be able to do that.

Is there a way to achieve what I want?