Don't think that this will now seperate enough for Workshop, and also not justify the price difference. You get the option to gain 2 $2s instead of the $4. That might be worth paying $4 for it, but as you need to gain two differently named, do you really want that? Yeah, you could get Estate/Copper in the endgame of a Gardens game, but otherwise? In the beginning of every other type of game, you would usually prefer Hamler over Hamlet/Estate or Hamlet/Copper. And oftern there is no different $2 on the board. So in the end it will play like Workshop.
Maybe just drop the restriction? So you can get double-Estate endgame moves, that will certainly be worth the $3->$4 cost increase. And you can get double-Hamlet, double-Haven when you want.