Hi Jan,
Unfortunately, the driver currently only enables the card if you do exactly what you did (plug a monitor in during the enabling operation and then you can remove the cable). For the FireStream 9170, the driver will be able to enable the card without a monitor attached.
Now, depending on your monitor, if it has 2 inputs and you normally only use 1 input, this is a trick I used in our labs. I plugged the second card up to the secondary input of the monitor to provide the driver with what it needed.
I haven't tried this myself, but a KVM might do a similar trick (I'm not sure what the KVM sends out on the unselected video port).
Michael.