I just checked and it says my BIOS mode is "LEGACY." I'm assuming this shouldn't be the case? If so, should I reformat my SSD and re-install as UEFI.
You also need to install Windows in UEFI mode:
Yes, you need to format the drive and install Windows in UEFI mode for multiple GPU's. Create a bootable UEFI Windows install flash drive using Rufus. Make sure to select UEFI partition for GPT disk from the drop down menu when creating the flash drive and boot to the UEFI installer from the boot menu.