Mmm.. I suppose if the source was "dual-link DVI-D", and the splitter cable simply separated the two DVI-D links (video outputs), then you'd get nice independent control.

My Inspiron 9300 notebook has dual-link DVI-D ouput, as do the high-end Apple computers. Not much else does, though.

Wikipedia has some great entries for DVI-D etc..

Cheers