How do integrated and discrete graphics cards work at the same time on a PC?

Question

Video cards

Good afternoon.
There is a PC with: motherboard Asus Z97-K, processor Core i5-4460, discrete graphics card Gigabyte GV-N670OC-2GD, two identical monitors. On the video card, one dvi connector works stably, the displayport works periodically (the image on the monitor disappears), the other connectors do not work (most likely physical damage).
I decided to try to use Intel HD Graphics 4600 (from the processor) and a discrete video card at the same time, connecting a monitor to each, respectively, and thereby get rid of image loss. Oddly enough, it worked. Previously, this seemed to be impossible. Dances with drivers - and everything works great.
Questions: how are the loads/resources distributed? If I start the game, whose resources does it use? The video card that the monitor on which the game is running belongs to? Is it possible to manually control the load?
Thanks in advance, this is the first time I've encountered this)