If you actually want to test with Steam, you do need two PCs. This is because the Steam client only allows one signed in user per device. You can’t have two clients running at the same time either.
You can run with -nosteam on the command line and it should work similarly using our NULL online subsystem (LAN based). But eventually you will have to deploy your game to multiple machines to test that Steam is working properly.
-game is used to force the editor into game mode, it doesn’t enable/disable anything related to online platforms.