I found a rather convoluted way to make DAIN windows use 2 or more GPUs. It's a bit of a hack but it's working for me. Here are the steps:
1. Extract 2 DAIN packages to 2 different folders. Call them DAIN GPU1 and DAIN GPU2 just for your reference.
2. Split the video you wish to process into 2 different videos.
3. Set the environment variable CUDA_VISIBLE_DEVICES with value 0. For this go to This PC -> properties -> Advanced System setting -> Environment variables -> System variables
4. Start the first DAIN (DAIN GPU1) with the first half of the video. It should start processing with the first GPU.
5. Set the value of the CUDA_VISIBLE_DEVICES environment variable to 1.
6. Start the second DAIN (DAIN GPU 2) with the second half of the video.
7. You should now have both GPUs working. As fas as I can tell this works for even more GPUs. You just need to keep adding the CUDA_VISIBLE_DEVICES value and launch more DAIN instances.
8. Join both resulting videos with a video editing app of your choice.
I hope this helps.