Dual Desktop mode on the H55/H57 motherboard is similar to NVIDIA's Dual View mode.  This requires that 2 displays are connected to the motherbord's video connections, one to the DVI connection and one to the VGA connection.  If a DVI/VGA adapter is used, the motherboard will automatically set the displays into a "Clone Mode" type of display that can not be modified.  To allow Dual Desktop, you must use a digital DVI cable from the motherboard to the display without any adapters.