You could either build your own API and send JSON data over the Network
Implement Gamekit and establish a MultiPlayer session to pass information between App instances.
The upside of making your own API is you have full control and don't have to deal with Gamekit (which can get very hairy)
but the downside is you have to maintain your own servers worldwide for the convenience.
So learning Gamekit (or at least toughing through making it work) might be worth your while.
The only other thing to keep in mind is that these coordinates can get skewed so you'll need a way to filter out ridiculous data.
ARkit will give you the coordinates, and you can send them over your preferred method.
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)