MArcomage

I think it's not possible to multiply enemy production.
I suggest something like:
production X2, enemy gets stock equal to enemy facilities.

Yes, something similar was discussed before. Your opponent cannot gain production during your turn. Fithz's suggestion would work, though, giving a simulated 1x production immediately in addition to regular production on the opponent's next play.

Changed the concept card's text following Fithz Hood suggestion