This is listed as a known issue. I think in this case there is no solution, and is the main reason I wasn't even going to add parallax in the first place. Technically, the sprite can only be in one location, so two viewers isn't possible currently.
I ended up adding it anyway, so that people in single player can at least enjoy it. If you're looking at different platforms, it should work correctly at least. You can also disable parallax without needing to close the server if it gets really bad, and re-enable it when you've finished doing work on the platforms together.
(Also if you do start turning parallax off and on while running multiplayer, let me know if anything breaks? It shouldn't, but would be good to check.)
