Steam - Need to host twice to be able to join

Hey, i set up a simple blueprint host / join server (by a serverlist) and whenever i click Host a game, i join in, but my friend doesnt see the server. If i start the game another time (2 instances at the same time) and press Host, my friend is able to see 1 server.

Why doesnt it work if i only have 1 instance open?

Do you make sure to destroy your current session before attempting to host one?