You are right, there are a lot of different HMDs around. I will soon post a survey to ask about which HMD you would prefer to use for VR Editor.
I assume you know about my plugin (MultiEdit), because you asked me for this. Epic doesn’t support that plugin. The feature you describe is pretty hard to achieve, so if I were you I wouldn’t expect it.
I don’t know if we are going to support the hydra, since you can’t really buy them. Our first priorities are the Vive and Oculus, because of their motion controllers and they are supported really well in UE4.
I think that kind of resembles to your question and my answer above? I do think a spectate feature would be awesome! If you are talking about running the VR editor twice or more on one machine, that wouldn’t be do able.