GX620 Dual Monitor support with VGA and DVI

Hi, I'm trying to get a dual monitor setup working on a GX620. Windows XP shows there being 2 ATI graphics card in Device Manager, I presume this is because its capable of dual output. I read that the card only supports DVI and VGA, not dual DVI or dual VGA, so i bought a DVI to DVI & VGA splitter and hooked one monitor up on each.
Windows XP boots and diaplys the BIOS post screen on the VGA monitor, but once past the XP Splash screeen, the VGA output goes blank and the domain logon screen appears on the DVI output.

I then sign in, right click the desktop, properties, settings to adjust the display options. Windows shows the 2 screens and I click to set the DVI as my primary and to extend my desktop onto the secondary VGA monitor but it doesn't accept the settings and won't allow me to scroll onto the second monitor.

I know the cable is working as during the boot process the output switches from one monitor to the other, but I'm struggling to work out how to use both monitors as an extended desktop..

Video settinsg are set to AUTO in the BIOS, so the Video adaptor is overiding the onboard VGA.

Any suggestions or advice would be appreciated

cheers

Jon