I've had EnableUlps set to 0 for quite some time (Crossfire related problem with 1 card randomly going to 0% load). I can confirm that this will NOT allow you to start mining on a GPU that doesn't have a monitor attached.
My solution. My monitor has 3 inputs. I plug my 2 cards in to it. 1 DVI 1 DVI-VGA. Start mining and overclock 1st card, hit switch input, start mining and overclock 2nd card.
Did you set it up on all cards eg. under the 0000 and 0001 or 0003 keys? It works fine for me, my config:
Windows 7 x86
HIS Radeon 5850
ASUS Radeon 5850 DirectCu TOP
Latest catalyst drivers.
CrossFireX enabled
Cards are linked with CrossfireX bridge. If it still doesn't work, there's another solution, run furmark or another GPU heavy application for a minute, then the second GPU will appear ready for mining - I was doing this before learning the ULPS trick.
This did not work for me either. I had changed all 3 keys to 0, but my cards are not linked by a CrossfireX bridge. I expect the CrossfireX bridge is the key here. If so, then I will stick with a KVM, as crossfire has a detrimental affect on the mining performance.