I did a similar setup on my 2021 Sierra except I used the commonly posted Pasternack extension cables and a cheap FAKRA splitter cable found on eBay/Amazon for about $10. When using the cheap FAKRA splitter cable the truck recognizes that a camera is connected but I can only get a black screen from it. After seeing your post I ordered the same cable from vcablemart as you did and unfortunately get the same results with it. Through extensive process of elimination I have confirmed every other part works properly (I have 3 GM trailer cameras, about 6 cables, 2 FAKRA splitter cables, and 2 GM camera/7 pin trailer plugs). Every camera works in every port with every cable as long as neither splitter is inline. With either splitter inline in any configuration, I can only get a black screen... even if I only connect one half of the split cable to either GM 7 pin/camera port receptacle and leave the other half of the splitter hang free. I'm at a loss as to what to try next other than give up on the hope of having 2 working trailer camera ports.
Could you confirm whether or not you have tested your setup with the vcablemart FAKRA cable splitter inline yet? If it works for you then I will probably continue troubleshooting mine... even though I'm not sure what else to try.
