Coherence and multiple displays

I have a macbook and an external display. The macbook resolution is 1280 x 800 and the external resolution is 1680 x 1050. I run Windows under Parallels in coherence mode on the external display but would like to run it on both displays. When I try to use multiple displays I get the following error:

"Unable to switch to Coherence Mode. Primary operating system does not support screen resolution requested by guest operating system. Select another resolution for the guest operating system or review your video system configuration."

I have tried reducing the Windows resolution to match that of my macbook but to no avail.

Does anyone have any tips for configuring this setup?

 

MacBook graphic card doesn't support big resolution in Coherence on two displays. Try 800x600 on both displays - it will work.

 

Thanks for that.
Unfortunately 800 x 600 is just far too low resolution to be working in, so I'll stick to running coherence at 1680 x 1050 on my external monitor.