I recently installed another GPU on my system. It's of the same type and driver that my other GPU is, and it is PCI whereas the other one is AGP. They are both running and detected, and I am wondering what exactly are the benefits of having two GPU's like this, don't be afraid to get technical. Thanks!

Let me start off with this:

To make use of Dual GPUs they need to be:

1. The same Model of GPU as the point of having another GPU running is
   to increase substantially the 3D performance or the Computing
   capabilities, in some cases motherboards support the use of different GPU models but it kinda defeater the purpose of having Dual GPU.


2. Both need to be in separate PCI-Express slots, so that you can get the most out of them.


3. You need a Motherboard that actually supports having this feature.


4. You need software that can make use of Dual GPUs.

Now I've marked part of your text in Bold because you say you have two GPUs, One is PCI and the other is AGP, and while both are recognized by the OS, you're not actually in a true Dual GPU setup, however I figure out that you can run different xservers on them as told here. Or use them for Video editing, or 3D modeling etc.

Still if you don't know what you can do with them, just because you "could" is not a valid reason.