Gonna get me triple head, if you'll excuse the pun!!

With regard of using two screens
I have a nineteen inch { identified no 2 } and a seventeen inch { identified no 1 }
Now regardless of which socket used, the 17inch screen is not so bright: yet wired alone on either socket the screen is bright.
Does one screen draw more currant?

Apart from purchase of two identical screens {oh and I have adjusted contrast and all controls} any ideas?