I just ran into this problem that drove me nuts, to permanently disable that crap look below.
http://www.overclock.net/amd-ati/667144-crossfire-disabling-ulps.htmlULPS saves power by sorta disabling the second card when it is not needed.
Here is how to do it:
Go to registry and disable Ulps:
[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Cl ass\{4D36E968-E325-11CE-BFC1-08002BE10318}]
and change "EnableUlps" to 0 under the 0000 and 0001 or 0003 keys.