I am having the same issue. I have more than one HDMI to USB converter of the same brand. I can't get them to work on the same USB 3.0 hub together. BUT...if I put one of them on a (separate) USB 2.0 hub, it will work.
I realize many have said this is because of bandwidth issues. But, please consider these points:
1) USB Bandwidth: Since the device works on USB 2.0, the bandwidth can't be more than 500 Mbps. So, two of them would not require more than about 20% of the USB 3.0 bandwidth, which is around 4800 Mbps. Note that the adapter in question is 1080p30.
2) No problem when using devices that are NOT identical: I can connect 5 cameras (that's all I had available for testing) without any issues if they're not the same model/brand. The PC in question has only 3 USB controllers. So, it's clear that more than one camera *can* be connected to a single hub.
So, I would like to put forward an alternate hypothesis: This is a naming conflict. Maybe having three devices named "USB Video" is just as confusing to OBS as it is to me when I'm trying to select one...unless they are connected through different USB hubs/controllers. The difference in USB hub/controller means that the connection is logically distinct, even if the device name is the same.
If my hypothesis is correct, the solutions to this problem would be very different. For example, we could look for a modification to OBS that automatically assigns sequentially numbered names to identical devices. Or, we could find a solution for renaming the devices. Or, we could develop a database of chipset use by brand/model, to aid users in buying multiple devices that won't conflict (I recently bought a device of a different brand with a different case in an effort to sidestep this issue. Unfortunately, the Win10 device name and drivers appear to be the same.)
Any thoughts?