Press Apply or Ok and close the window. It will still run but hidden. Now go to your Nostale Icon and right click on it. There you will see ''Run Sandboxed''. Click on that and Voilà, you have your Nostale, now just open Nostale like you always regularly do without sanboxed. Now you have 2 Nostales running!
BUT YOU CAN'T LOGIN IN THE SAME ACCOUNT ON BOTH OF THEM, YOU WILL NEED DIFFERENT ACCOUNTS.