- for this, you need two devices connected to the same network (e.g., wifi) with the same game progress.
One is yours and the second is a friend's. Then they can see your game in their game center just like you see theirs.
There might be problems related to network connection between devices, but it shouldn't be a massive problem if you both have a decent internet connection speed (at least 200kbps).
I'm playing knife hit mod online with my friend all the time on both android and ios devices.