I think the biggest problem with introducing duo cards is the UI changes involved with making them.
It would imply needing many, many, combinations of new cards to effectively make itself a part of the game. And in the current game, the introduction of even one card from all the Duo combinations would swamp out the entire rest of the card pool, so I say not yet.
Maybe these could be added into the game when the card pool gets bigger, and we can make a big project out of it, but as the game currently stands, there's no point in adding 1 or 2 duo cards when mono cards work perfectly fine.