The short answer is, you can’t. There is no way to share a datastore between two separate games.
The only way you can effectively transfer data between two games is by using some sort of external tool or service to store or transfer that data, or, hosting your own.
However, you can avoid the need to transfer data like this by simply using the same universe for both environments.
Personally, the way I would set up a test place/game is by having the “test game” teleport to a “test place” in the main game. You might have a second place under your main universe (aka game) and your test place where the players join from can teleport them in to the second place.
Even better would be to use the same place but pass some sort of test mode parameter over the teleport to tell the server to start as a test server.