PCIe x16 and dual x8 new and confused - CPUs, Motherboards, and Memory - Linus Tech Tips
The CPUs doesn't have unlimited PCIe lanes, for example mainstream Intel CPUs have 16 PCIe lanes and Intel "enthusiast" CPUs(X79) have 32 PCIe lanes(if I remember correctly). So if you have 2 GPUs in your system that are PCIe x16 and you have 16 PCIe lane...