I imagine that Goodreads is able to do it because in the underlying database people add editions to their shelves rather than books - the editions are just associated with a given book. In the absence of true edition tracking in Grouvee, the platform flag is also somewhat analogous (though probably fails in some cases). Upon splitting of a page, users could be distributed the two child pages based on selected platforms, however this is problematic because platform selection is presently not mandatory, so I don’t know how many people actually select platforms.
A temporary way to facilitate splitting could be to create a new page for one of the games as you mentioned, and leave all users on the original page. In update emails (like the “New games from Steam added to Grouvee!” ones I get when I buy a Steam game) users could be notified that one of their games has been split, and that they should double check that they are associated with the one(s) they want. But this still has the problem of not directly migrating reviews, comments, updates, and discussions, and thus kinda breaks the social features. However, this problem only gets worse the longer one waits to address it.