Shared photos is likely very doable, as it's just photo id, physical location, and scale/orientation. Place a picture above the fireplace and both headsets could render it exactly the same and see it.
Video would work well, audio might be a little harder to sync exactly correctly.
It shouldn't be much more complicated than what we currently have with first-person 3D shooters. All devices in an augmented room would have some existing shared data that doesn't need to be transferred. It's only the updates that need to be sent over the network.
Video would work well, audio might be a little harder to sync exactly correctly.
Apps are going to be really really hard.