Windows 7 and 8 on separate disks, or dual boot?

I currently have Windows 8 installed on an SSD. I need Windows 7 to be available on the same machine. If I just change the boot drive in the BIOS, can I install Win 7 to the new boot drive without any problems? That would require changing the boot drive in the BIOS to change Windows versions, but that would probably be ok.

Alternatively, given that I have Win8 already installed, what do I need to do to install Win7 in a dual boot configuration? I think all the notes I have seen about dual boot were about adding Win8 to a Win7 system, but I need to add Win7 to a Win8 systtem.

If you Upgraded from the Windows 8 upgrade.

You will need two Windows 7 license & one Windows 8 license to dual boot.

My question is technical and not about licensing--are there any gotcha's about adding Win7 to an already installed Win8? I have the necessary licenses.