Is your vid card an original card for that PowerMac?
I don't think so, as the original card would have the Apple ADC connector (in addition to the VGA), which is a DVI connector that only Apple used.
The ADC connector is oval, not rectangular.
When you mentioned dual or single DVI, then you must have an aftermarket card.
If it was NOT manufactured as an Apple card, then you (or someone else) "flashed" a PC graphics card to use in the Mac.
PC graphics cards won't work in Macs at all, unless the ROM is designed for the Mac, or the card is flashed for use on the Mac. There's just a small number of cards where that works, and the resulting card is often unstable on the Mac. That would be another explanation for the non-support of the wide screen display on the DVI output, as the flashing software may never have been a complete changeover - perhaps leaving out some functionality that was originally provided for a Windows PC.