I play Terraria a bit with my brother and friends and I am always the one with the server, but sometimes while I am playing it gets a bit boring and I want to play on my own world and character (I do know about that whole duplicating cheat and am not a fan) at the same time. I was able to do this for a while and now I can’t. Is there a setting that I accidentally turned on or just an annoying patch?
A good way to do this is to run a Terraria dedicated server. This is a separate executable that runs the server in the background, leaving you free to launch Terraria and connect to the server, or play single player while the server continues running for everyone else. To run the dedicated server, browse the Terraria folder in your Steam folder, and run TerrariaServer.exe. For further information, and to set up port forwarding if required, visit the Dedicated Server page on the Terraria wiki.