This is not made obvious at the outset, but a video card with a DMS-59 connector that can drive a pair of Single Link DVI monitors cannot drive a single Dual Link DVI monitor: the DMS-59 connector doesn’t have the required signals.
The fact that each of the Single Link DVI connectors on the end of the DMS-59 Y-splitter cable have enough pins (well, holes) to mate with a Dual Link DVI cable doesn’t clarify the situation.
Of course, you can’t combine two separate Single Link outputs into a Dual Link input.
An HDMI 1.4 cable can support a single 2560×1440 monitor, but not from this resolutely low-res video card.
The Smell of Molten Projects in the Morning 