You need only one card present to flash it.
Make sure you backup the old bios prior to flashing.
If the flash goes bad put both cards in the computer or put a PCI card and reflash using the old bios.
Here's another way for the 8800 GT.
Download the latest rivatuner and Quadro drivers. Install rivatuner and the geforce drivers that came with the card, then reboot and follow these steps:
1.) Open rivatuner
2.) On the upper selection area look for "Low-Level system settings" and open it
3.) Go to the NVStrap Driver tab.
4.) On the PCI DeviceID settings look for a dropdown box that says "Graphics adapter identification". Select "Custom" there.
5.) Cycle through the new options that you'll see now, until you find one that says (NVIDIA Quadro FX 3700). Press apply and Ok.
6.) Reboot your system.
7.) Uninstall the Geforce drivers and use Driver Cleaner Pro to make sure you got them all.
8.) Install the latest Quadro drivers.
9.) Be the happy owner of an FX 3700 for $120.