I'm having a similar problem. I used to have my PC monitor and an HDTV connected to my PC at the same time via a VGA splitter -> DVI to VGA adapter. Having gotten a newer TV, I realized that I can get higher quality with a DVI to HDMI cable.
I just got a Y DVI splitter, basically trying to do the same thing. From the PC, it's got the DVI splitter plugged in. In the splitter is connected the DVI to HDMI cable (to HDTV), and DVI to VGA adapter -> VGA cable (to PC Monitor).
At first I thought it was broken because only one picture showed, but it seems that, for some strange reason, it will only show the first input plugged in! If I plug in the DVI to HDMI, it'll show on the HDTV, but plugging in the DVI to VGA adapter does nothing. Same with vice versa.
Any and all help is appreciated.