Use Steam VR-enabled applications with your phone as HMD (Head-mounted display). The only Open-Source solution to similar commercial packages like VRidge, iVRy, Trinus etc etc.
I would like to suggest the possiblity of using the application with an USC cable instead of using a server.
Streaming is pointed out nearby as a suggestion.
So multiple phones can see the same thing, but only one phone controls.
I would like to extend this idea in order to make it easier to implement regarding third party servers.
My solution would be that the view-stream provided to a controlling phone should be available to devices connected with a cable.
I would be able to plug in my phone to the pc the other person is playing on, and see what is he playing
I am going to call this scenario "join with cable"
Joining with the server is also an option to implement, but personally i do not recommend to try that, because it seems more complicated, time requiring, and possibly detrimental to performance
I would like to suggest the possiblity of using the application with an USC cable instead of using a server.
Streaming is pointed out nearby as a suggestion.
I would like to extend this idea in order to make it easier to implement regarding third party servers.
I would be able to plug in my phone to the pc the other person is playing on, and see what is he playing