i did when i launch my miner one worker is device 0 and the other is 1. when i disable crossfire no matter what i change the device to it either uses the cpu, or does not run at all.
You must be using Windows. You need a dummy VGA plug:
http://www.geeks3d.com/20091230/vga-hack-how-to-make-a-vga-dummy-plug/. Windows will not enable the GPU unless it thinks there is a monitor installed.