I’ll check why you are not able to run two copies of the program at once, as there shouldn’t be any issue around it. If you’re running the Steam version, the first one will block the SteamApi and you will be unable to start a second one, however this shouldn’t be the case with the version downloaded from our site.
Can you inform us how the program behaves after the restart? I would be tempted to know how it gets stuck after quitting in the first place.