It appears that the Mac Pro exhibits issues with video display when two graphics cards (the problem may be manufacturer/model specific) are installed in a configuration where one card is running at 16 lane speed, and the other at 8 lane speed (the maximum throughput possible in a two card configuration). Issues include an inability to display graphic content in some applications, and a lack of screen refresh.
The solution is to re-configure the Mac Pro to use both cards at 8 lane speed, which will eliminate refresh and display problems, but also cause slower graphics performance.
Lane speed configuration can be controlled with the Expansion Slot Utility, located in /System/Library/Core Services.
MacFixIt reader Frans van der Geest writes:
"I've installed an extra NVIDIA 7300 (bought through the Apple Store) in slot 4 running at 8 lanes PCI-E speed. The standard built-in card runs at 16 lanes. In this setting iMovie does not show content when playing scenes but instead a white screen. Also third party software (like Boinx FotoMagico) does not update screen. Applications like Safari, QuarkXpress 7 etc. do not update screen sometimes. Changing both cards to a 8 lanes setting solves does but slows down one card. Also 16 lanes and 4 lanes does not work as 16 lanes with 1 lane again does."
For further coverage, see our Mac Pro special report.