Welcome to the community, @bmann97!
Since you are most probably planning to install the OS onto the SSD, I'd suggest you plug only that drive to the motherboard and attempt to install Windows on it. If you have both drives connected while Windows is being installed, you might encounter an OS confusion.
If the SSD still doesn't pop up, try
resetting BIOS and then make sure you configure the SATA mode for the SSD to AHCI. Another thing you should check is your motherboard manufacturer's official website and make sure you have the latest SATA controller & Chipset drivers installed. While on it, you should also look at the latest BIOS version, because you might need to update it.
Hopefully, that will fix the issue. Keep me posted, though!
SuperSoph_WD