To scroll in the middle of two players use lerp(player1.x, player2.x, 0.5)
If you need to zoom out when players are far away from each other, you can set layout scale to something like this:
min(1, (ViewportRight("HUD")-ViewportLeft("HUD")) / (abs(Player1.X - Player2.X)*1.1))
Where "HUD" is a layer with Scale Rate=0.