Hello and welcome to CentOS fora.

I advise some necessary reading for all new members:

How To Provide Information About Your System
Installig Software
Readme First
Where To Find Answers


Now, regarding your problem:

For start, I suggest you remove both the dkms and the driver from nVidia website, and replace 'em with the one from the ELRepo repository (package kmod-nvidia), but configure the Yum Repository Priorities first, of course.

ELRepo provides a kABI nVidia driver which supports your hardware, is kernel independent, packaged as an .rpm, and with it, you won't have to reinstall it every time you install and boot a new kernel.

Sadly, I have very little experience with two monitors attached to one VGA card... and have never witnessed what you're describing.

But regardless, for start I warmly recommend you replace these drivers with the one from ELRepo - as you can probably see the benefits yourself.