This was my concern, too.

When new players sign up for the game after the date of the fork, we plan to create their account on both the Steem and Hive blockchains so that they will be able to use and interact on both chains outside of the game using the same account name and keys.

Doesn't help if they create their account another way. We almost need the front ends to check that any account buying a card, pack, orb etc has the same public posting key on both chains, before creating the transaction.