There is a mode, on FirePro graphics cards, called SST - Single Select Timing and on our 14.xxx drivers that is enabled.
When two passive adapters, DP to DVI, are used and identical displays are attached which don't run higher than 1280x1024 res then SST remains enabled and as a result there is no display on boot into Windows. If an active adapter is used alongside a passive adapter then SST is disabled and system boots with Windows displayed correctly.
This should be resolved in the new AMD driver 15.201