Harold, this really is NOT a solution. I have just upgraded my PC with a fresh Decklink Duo2 card, and installed it with latest 12.4 Desktop Video bundle from Blackmagic. It simply does not work with OBS, and is a OBS bug, that should be fixed.
After few hours trying to figure this out, I have come to the following conclusion on my setup:
1) If you set the decklink card to group SDI1 & SDI2, then video capture ONLY works with SDI1 input. No other input is possible.
2) If you set the decklink card, to map Decklink device 1 to 4 to SDI1, SDI3, SDI2 and SDI4 respectively, I can easily confirm using the Blackmagic Media Express software, that all inputs are indeed working. But in OBS, none will work. I have only accidentally discovered a workaround:
In the Properties dialog for the Video Capture Device, under the device selection, there is a Configure Video button. Press that, some generic Windows dialog pops up, where you can select "Override Video Mode", set it to (in my case) 1080p60 and voila, after pressing Deactivate/Activate at the video source, an image appears!
This way, I can make any input work and appear in OBS.
So, it seems, that a video format detection does not work right for the decklink inputs. I would be interested to see, what would happen if
Steve Dubrava could try this workaround.