I'm mining with on Windows my two 5850's and didn't ever thought about need to use the dummy plugs, it isn't needed!
You just need to disable ULPS - this mechanism disables the second card without monitor/workload. Here's how to do it:
Go to registry and disable Ulps:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Contro l\Cl ass\{4D36E968-E325-11CE-BFC1-08002BE10318}]
and change "EnableUlps" to 0 under the 0000 and 0001 or 0003 keys.
Sir, you're a gentleman and a scholar. I'll try that later. Although I spent $16 and ordered 20 vga/dvi dongles and 100 capacitors to make the ghetto rigged dummy plug.
Not that I don't love having hdmi-hdmi for my main system, hdmi-dvi for gpu1 on miner and vga-vga/hdmi dongle - hdmi for gpu2 on miner
