There is a lot of debate surrounding the topic of PCIe slots and bandwidth. Some believe that PCIe slots do not share bandwidth, while others believe that they do.
The truth is that it is difficult to determine exactly how much bandwidth each PCIe slot is using.
Some factors that could impact the amount of bandwidth used by a PCIe slot include the type of graphics card installed in it, the number of devices connected to it, and the speed of the devices connected to it. In theory, if two graphics cards were installed in adjacent PCIe slots and both were operating at their maximum speeds, then each card would be using 100% of the available bandwidth.
However, in practice this is rarely the case.
Some video cards have multiple ports that are capable of operating at different speeds. This means that one port on the video card might be using 80% of the available bandwidth, while another port on the same video card might be using the full 100% of the available bandwidth.
This means that two graphics cards installed in adjacent PCIe slots would likely use significantly different amounts of bandwidth – one might be using 40% of the available bandwidth, while the other might be using 60% of the available bandwidth.