How the fast card takes a hit in processing to keep in sync with the slow card. I'm going with nothing wrong here. You should avoid mixing GPUs like this.
Recently i put a second graphics card into my computer (just a GTX645) and everything ran fine, however all of a sudden i'm noticing when i have a window open watching Youtube my main screen looks to be having issues with lag. No frame drops in game but I have a 144hz monitor and I can tell its not 144hz at that moment. Settings don't change but i don't know the problem. I'm thinking more ram but why would that effect my screen. I have the right cable for a 144hz screen too. Idk why it just stared happening. it ran fine before.
CPU: Intel I5 4590
MB: Gigabyte B85M-D3H
RAM: 2 4GB DDR3
GPU #1: GTX 960 GPU #2: GTX 645
POWER: Corsair 450M