Yes. Each card must be done separately.
Unless you are booting Windows 8 in Secure Mode you don't need the UEFI BIOS anyway. You can tell in Disk Management as the C-drive should be configured as GPT. If it's not, you aren't in Secure Mode.
I doubt you're in Secure Mode because I believe the graphics cards had to be UEFI compatible PRIOR to installing Windows. There's also a couple BIOS settings that if not correct cause Windows to install in Legacy (non-secure/UEFI mode).
*My computer started freezing during boot after I flashed the BIOS. It may be a coincidence. I reinstalled Windows 8 and it's fine now.
So again, unless you are planning to reinstall Windows 8 in UEFI mode I don't recommend flashing the BIOS.