Four player split-screen. And if MonoGame updates Enum_PlayerIndex, eight/16 on Steam, Windows 10, XBoX OnE. Although I may just have to remove it from XNA's GameStateManagement, useless however, as you still need the specific platform or devices.
Either way, the feature doesn't appear to be built into XNA or MonoGame? And considering you can only have one extra headset, it's too limiting to my specific split-screen game.