If you can solicit team members, TestFlight is your friend.
Otherwise, don't ship something your users have to test for you.
Well in the future hopefully Apple will get ARkit working in the simulator.
In the meantime, I would suggest "emulating" whatever AR objects and data you have in a non-AR Scene, then your testing would be reduced to simply seeing whether ARKit is capable of running smoothly in the context you have created.
This way, you can test the Game Center functionality using your phone and the simulator, and save the ARKit Testing for when you actually have access to another device or get someone to help you test. (Either by coming over or using Testflight)
As for your "second user", just create a normal Apple ID and sign it into GameCenter on one of your devices.
You only need Xcode to be signed into the Dev ID program (For code signing purposes).
I just read about ARKit 2 facilitating a multi-user experience.
This might mean that Gamekit is too slow for AR even without network lag and wouldn't work, in which case my workaround won't work at all.
Then again, things change when you're actually in the thick of development, perhaps theres a chance it will.
(I'd love to give you an answer but this testing would require quite a bit of time)