If you’re looking to do something similar to NVLink, you’re out of luck with the 4090. Aside from the massive power draw, there is no benefit in having two of these gargantuan GPUs for gaming purposes. You can’t readily tap into the extra GPU thanks to sunsetted support for NVLink.
The 3090 is the last flagship from NVIDIA capable of NVLink, which again still requires a PSU with a massive rating to run both in series. In all honesty, the 3090 and 4090 are more than capable of maxing out every recent game on the market. Even newer releases like the Dead Space remake are no trouble getting maxed and looking great.
In the event you’re looking to run multiple 4090s for AI or ML purposes, you certainly can. Certain workloads can leverage processes on multiple cards in parallel.