Actually incorrect, on BeamNG's vulkan API it supports "SLI" or "Crossfire", but it's not utilizing that ancient technology. Vulkan API actually takes advantage of more than one graphics card whether it's AMD or Nvidia. It's not considered, well, SLI or Crossfire anymore though. People also forget that on average the Vulkan API is <~+20 FPS ontop of standard DX11 as it's a much better renderer. (Unless you're maxing parked vehicles at 50.) An odd example of getting the Vulkan API to utilize two GPUs, I took one 1080ti and a 6900XT and slapped them into the same motherboard. Now this wasn't an immediate net-gain or anything, it required some work to utilize the Vulkan API to take advantage. But once it did, we encountered a multitude of other bottlenecking issues. BeamNG's performance is VERY rarely bound by GPU. We saw no significant or advantageous use for multi GPU utilizing Vulkan API. The cons of the utilization did not way the benefits. On Utah, with a 5950X and a 3090ti with a 12 vehicle, 12 pool, and fifty parked vehicles we averaged a continuous 24 FPS on non simple vehicles. On a 13900K with the same 3090ti and settings we averaged 24 FPS. Turning on simple vehicles and dropping the parked vehicles down to 4, both computers levied 120 FPS on average with a overall 1% of 24 on Utah. With simple vehicles off, our average FPS did not change, at all. But also take in mind we're using the Vulkan API. On the DX11 renderer, our FPS dropped significantly on the lower parked vehicles setting from an average of 120, down to a solid 70. We had no change with 50 parked vehicles, and they levied the same FPS whether it was DX11 or Vulkan. All these tests were unfortunately in Windows 11, and not 10. Also take in mind BeamNG does not utilize all 32 threads, or 24 threads of both the 5950X and 13900K. In terms of price<>performance, you'd achieve similar results with CPUs significantly cheaper, by at least a few hundred dollars. 13700K, and 5800X3D are good examples of this.
I´m even more late to this, but, I had RTX 2070 with Ryzen 9 3950x (16 cores) and I could run around 15 cars with 24 fps +- .. Recently I switched to RTX 4070 with Ryzen 9 7900x + 64GB of ram, I get 35 FPS with this setup. Not as much difference as I expected, but better than 24 fps.