One eye minimum needed for NVG, two gives you some potential for depth perception. Four is for peripheral vision but most don’t need that pilots or special forces commandos need that for very close fighting.
The problem is that if you mount on the helmet you can’t use rifle sights. To get around this you might mount a laser but in the process if the Opfor have NVG they can see that laser and shoot at you.
If you mount on the rifle you can’t use helmet sights. But mounting on the rifle means you don’t need the laser, however you loose any potential depth perception or peripheral vision and can’t use the NVG for anything but shooting.
The set up we saw at the Parade seems to try and work back some compromise by a very compact NVG monocular and a rifle mounted system. the small monocular being small enough that the shooter could may be mount it and bring up the rifle NV sight.
The other scheme to work around this is what the US is trying with the ENVG II and FWS I where the weapons sight streams data to the goggles in either binoculars or Monocular configuration which appears like a video game crosshairs.