Is it possible to use the same key for 2 different websites, if so what are the differences between using the same or a separate key?

Yes, it's possible. If you use 1 SEON account you can see users across both platforms, by sending transactions of both websites with the same license_key.

If you use 2 accounts, everything is separated between the two Admin panels and their respective databases. You can have separate billing, settings, and databases this way.