From your descriptions of horizontal and vertical, the horizontal one at the bottom is your graphics adapter, the vertical one is on-board.
most of the time, you can't use both. using a Graphics card will disable the onboard video.
VGA is old school anyway (guessing you're using VGA since you said the plugs were blue) and I strongly advise anyone to move away from it, even DVI is a world better. Sometimes boards without on-board graphics or that pre-date graphics capabilities included on CPUs will have VGA on board, because Windows natively contains VGA drivers and you can use it to build/image/whatever a PC.
You may need to think about changing the graphics adapter if it only has one usable video-output on it.
Random pic pulled from google and doodled on to illustrate what I said:-
To asses your requirement, what size monitors are you using? VGA only really supports 1200ish by 720ish before you start seeing tearing and flickering because it can't keep up with the data rate.