I tried taking a stab at this a while ago because I couldn't do it so I deleted the events for it.
My game is 800x600, and the player is a 50x50 sprite fixated in the middle. But if someone presses start on a second controller. Player 2 will pop in.
But then I want the camera to switch modes so it doesn't just follow Player 1 anymore. So if both sprites are within a 400 pixel radius of each other it will be like single player but if both players are more than 400 pixels away from each other then it will zoom out to compensate. I want the game to rely on teamwork when using splitscreen so if you get too far away it'll be hard to see your character and enemy / enemy bullets. But when I try to zoom it zooms my HUD out too (I have my HUD on a separate layer with a parallax of 0.) and even the smallest change zooms the camera really far out.