If you have one of those laptops with desktop cards you cannot expect to run those at their max capacity. Most of those laptops are not designed/engineered correctly to keep up with cooling. So 2 x 4K displays are a nonstarter, let alone trying to render at the same time.
I have a Precision 17" (excellent cooling design) with a Quadro RTX 500o and I could not run 2 4K displays on it reliably. I now run the 2 4K displays with an external eGPU and it all works so well: eGPU on Windows Laptop. First tests - #3 by gustojunk
I suggest you go back to only one external 4K, or you do renderings, but not both at the same time. I hope this helps.
G