Are the data in these two documents always to be linked, so that updating Doc1 updates Doc2? Or is Doc2 just supposed to get the data to populate its own dropdown from a list in Doc1?
If it's the latter, rather than using a separate document (though that is possible), you might consider holding the dropdown data in an Excel workbook and retrieving them from there. This is especially useful if you already have the data in an Excel workbook that you use for other purposes. See, for example:
https://www.msofficeforums.com/word-...html#post46287
https://www.msofficeforums.com/word-...html#post97965