PCIE 5.0 16x split into 2x8 with riser cable

Hey guys

Thanks in advance for your help and knowledge!

My setup is born out of the parts I had at hand. Wanting to maximise VRAM with an RTX 4070 that I had in another system that I only used once or twice a year.

So right now my system is 14600kf, 32gb RAM and rtx 5070ti 16gb on pcie 5.0 16x, and a rtx 4070 on a pcie 4.0 16x slot that runs over the chipset of my z790 motherboard.

I know that my cpu has only 20 pcie lanes and that this is not great, for what I'm doing, which is why I'm asking, if any of you have experience with splitting the pcie 5.0 slot into two.

The use of the system is twofold, during the day, the I want this to run my Hermes Agent (gemma4, 26b and qwen)and on a Linux OS and during the evening I want to play games on it with windows.

So far Generation speeds are good as long as context is 16k (3s) once I up to 128k generation speed for for example OCR is getting slow. (10-15s)

Would a split riser solve that? Or just suffocate my 5070ti?

I know gaming performance would be impacted a little by 8x.