you keep using the word mmo, I don't think that word means what you think it means
SS would make the game no better or worse IMO, because its unfair in ways, a 3 team Strike fire squad for example, your console would control 2 of the 3 slots, if you lose connection, the guy you are playing with is screwed. Also Hardcore players avoid SS users. Also both SS users would have a vision handicap not being able to use the whole screen.
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Just make them into their own fireteam and nobody will have to play with them, unless they choose to accept the invitation.