Recommendation on two or single GPU setup

I will start off by saying that I am no expert in PC hardware specs. I do basic research for what I want to do to make sure everything is compatible and go from there. Forgive me if any of my terminology is off.

That being said, I built my own water cooled pc a few years ago with an NVIDIA GeForce RTX 2070 Windforce 32GB. I've decided that I want to upgrade to an RTX 40 series, however, since my GPU is tied to the loop, I will have to bend more hard tubing and get another water block. All things I signed up for when I initially built this pc, but of course now I'm avoiding all of that as well as the task of dismantling a new GPU.

I have an idea and I've ran it through ChatGPT as a reference to see it's plausible. To get around everything, I'm thinking about just adding the 40 series to have a two GPU setup. I would configure my computer to use the 40 series for games only and the 2070 for everything else. Now my motherboard would run both PCI slots at x8 instead of x16, but I don't think there will be any noticeable difference given how I am using each GPU? Now because there's no room for me to mount the 40 series in my case, I have purchased a vertical GPU mount that will sit on the desk and be powered separately by an extra power supply I have.

So, I'm curious if anyone has an opinion on if this and if it seems like an ok work around or am I just crazy and should and just run the computer with just the 40 series? Thanks for any help!

I attached a pic of my comp for reference.

Specs for other parts

CPU - i7-9700K CPU @ 3.60GHz, 3600 Mhz, 8 Core(s)

Motherboard - MPG Z390 GAMING EDGE AC (MS-7B17)