No, you may indeed lower their res. but you can't increase the total Hz unless you overclock them, and doubling the number of Hz would probably fry the monitor, so i wouldn't recommend that.
If i got right, you just want to change the settings for some games. That you can do inside the game's own settings, lower there the res. and you will achieve higher FPS in that game, still the max Hz of the monitor will stay the same.