Two oculus rift DK2 and two nvidia cards in same machine

I think that would be fairly easy to do with SDL2. You can create multiple windows each with it's own OpenGL context.

The Oculus Rift has usb cables for sensors and camera... Maybe causes conflict... How to distinguish between each other?

If the SDK supports it, you can usually tell them apart by USB port. I also noticed that each positional tracker has a unique serial number. I guess the DK2 has one as well. (For the Metaverse, this is the persistent super cookie that facebook needs... :? )

I think you will have performance problems, most games with up-to-date graphics have troubles just feeding a single DK2 with 75fps.

The SDK has support for reporting back separate headsets, although I can't say I've tested it.

As for the camera, you'd need two cameras most likely since the LEDs on the headset are synced to the camera's shutter. As long as the two camera-headset pairs sync at separate times, you should be fine.