it appears that if you run 2 game instances (Standalone game) while using the steam online subsystem, you will no longer be able to use it, nor will steam show that you are playing “Spacewar”. any answers will be appreciated.
The times I have fiddle with Steam Online Subsystem, I have found that I get the best results when I package the game and launch from Steam. Any tests that I do when the game is not launched from steam I do not trust. So if I want to test achievements, or if I want to test Leaderboards, then I package the game and launch from Steam.
That means that if you are doing some tests with multiplayer, I would get 2 different PCs and test like that, with each game with it’s own Steam account.
Packaging the game solved the problem, thank you 
How did you manage to package it with the plugin enabled? I always get an error.
To anyone struggling with this issue,
I had exactly the same problem. My steam integration was working perfectly fine for months and then I set Number Of Players in the Multiplayer Options section to 2 players and it stopped working when using Standalone Game Launch; even upon reverting it back to 1 player. It also seems to set your window size to be different too.
I didn’t think having to package the game just to get it working was a solution, as this could take a long time. Very annoying inconvenience for everytime you want to test your multiplayer!
Fortunately, I found deleting the files in the config folder [Unreal Projects/YOUR GAME/Saved/Config/] completely fixes the issue.
I think this is a much better solution.