Well, I know it's possible because I frequently use an AF and an MP on the same box at the same time. My suggestion would be to unplug both Orbits, uninstall the drivers via Add/Remove programs, and then install the drivers fresh from the CD or Web distribution files with no Orbits connected. Then plug in your Orbits one at a time...
Also, depending on which 11 series drivers you have, the problem may well be a driver bug. Pretty much every 11.x driver release had some bad problem or other. I found 11.7 to be reasonably stable, and now I'm running the 12.x drivers which are buggy but not fatally so.
12.x drivers are available on the Logitech support pages, only they don't call it 12. It's called "Logitech Webcam Software with Vid - Version 1.0" now.
You can probably still get the 11.7 drivers over at
ftp://ftp.logitech.com