Let's assume we have use case that can be triggered by two or more actors e.g. User1 and User2. It will have the exact same flow for both actors. How should I describe this use case correctly? Who is the primary actor - User1 or User2? Should I make two scenarios for both actors or make one and set two primary actors?
If two different users can use the system for the same goal, there is in fact only one use case that has one primary actor. You need to come up with a common name for the two users and create an actor that generalizes both users. Often this is difficult, because, other than having the same goal, both users might have nothing in common. In this case just use an artificial name that contains the goal. If both might want to use a door to enter the room, the common name could be "room enterer".