Um, I may be saying something stupid, but can't you set the higher resolution monitor as the one to be mirror from? I. E the primary? Since even if it's powered of it will probably show up as a display) (just a guess) if you lack ports on the gpu, you can try a cheap secondary card(although you will have to set the game engines, and other programs priority to the main gpu, which might be a hassle) or you can just use hdmi or dp splitters (price/value is a good question as you are running high refresh, which some splitters might have problems with)