Capable of utilizing more than one GPU in order to increase performance.
You still don't explain what you mean by "one GPU". I gave you an example. Mac Pro is the only Mac that can have two GPU cards like Vega VII. All other Macs have one GPU. So do you have a Mac Pro or are you talking about GPU cores, like M1 which has 8 GPU cores?
Since Mac Pro is the only Mac with "multiple GPUs" the answer is yes. Many games, if not all, can take advantage of Vega VII Duo.
If you're talking about an eGPU connected to a Mac the answer is no. You can't use both the eGPU and the internal GPU at the same time.
To the original question: not that I know of. Most games don’t even support HiDPI abs wide colors and you are asking for multi-GPU support
Some Mac laptops have two GPUs: the iGPU abs the dGPU. And you can definitely use them at the same time. Add an eGPU - now you can use three GPUs at the same time.
So Macs don't have something similar to Nvidia's SLI? I know that in other apps than games having two graphics cards like W5700X or Vega II Duo in Mac Pro increases the performance. I didn't find any gaming benchmarks and misread this X-plane benchmark. Thought it was Vega II vs Vega II Duo.
As for the multiple GPUs in the same Mac I now that many Macs like MBP can have Intel iGPU and AMD dGPU and the system can switch between to save power but in games they don't use both. If you play a heavy game like Borderlands or Metro the games uses the dGPU. It don't use the iGPU to increase the frame rate. The same goes for dGPU. When you connect it to your Mac you choose which GPU you want to use. To play a heavy game you choose obviously the dGPU and the game won't use any other GPU. Or am I missing something? As far as I know Macs can switch between their GPUs but not use them at the same time in games to boost performance.
You can use whatever GPU you want. The API allows you to use all GPUs simultaneously. Do games do it? No. Is it a good idea? In most cases, no. But you can do it if you are so inclined.