Are you saying the 2 worked with dvi+dvi but doesn't with dvi+hdmi?
I ask because I can't get 2 to work using dvi-d+dvi-i. Both show the bios messages and even the windows loading screen. But the digital monitor shows "no display signal" when the "Press CTRL-ALT-DEL to log on" message is displayed.
This happens in both XP SP3 and 7.
Digital monitor is connected using dvi-d. Second is an old vga crt and is connected to my card's dvi-i using a vga 2 dvi-i adapter.
This: http://support.amd.com/en-us/search/faq/170 is about 3 monitors, but maybe the part about the two dvi applies. It says for dvi+dvi+hdmi, the dvi monitors must be identical.