Mirror the list of one board on another
Magic Mirror List for Trello allows you to mirror the list of one board on another. Furthermore, if you drag a card to the mirrored list it will be transferred to list that it is duplicating (and thus be shown as a copy in the mirror list).
To get started, create a list on a board with a title containing the list and board to mirror, following this syntax: "[My list to duplicate@The board that list is on]".
Obviously you need to be able to access both boards, and allow the plugin to act on your behalf as described in the OAuth dialog.