Activating a dual boot install of Windows 10?

Since Windows 10 came out I tried upgrading and doing clean installs 3 times total and each time I had issues that made my system unbearable to use or some things like my network adapter just wouldn't work even with "Windows 10" drivers.

Today I decided to give it another go but on a completely separate hard drive, keeping my Windows 7 install this time in case it goes wrong again like I was expecting it to (dual booting).

Unfortunately/Fortunately the install worked perfectly so now I have an obsolete install of Windows 7 and an install of Windows 10 that I can't activate. I tried using my Windows 7 key and it came up invalid, possibly because it's still in use or because I didn't actually "upgrade" my current install.

I'm almost tempted to just drop the cash on a key since it's finally working, and I really don't want to go back and try an upgrade of my Windows 7 install because that method has never worked before and I doubt it would now. I have a working install of 10 and would REALLY like to just leave it at that.