1. Have 2 copies of the game, each from a different region. Example: North America and Europe
2. Send an invite to a friend, or try to join on a friend through the PS4 party chat.
The player who tries to accept the invite will receive a message saying "Can't find application, do you want to look in the PlayStation Store?" Furthermore, you will receive the same message if you try to join this same friend through the PS4 party chat by clicking on their name and selecting "Join Session"