you need:
-2 counter objects (s x and s y)
-two players (p1 and p2
-an active object for the scrolling

Events:
every 00:00:
-[set counter(s x): x(p1)-x(p2)]
-[set counter (s y): y(p1)-y(p2)]
-[set x position (scroller): x(p1)-vaule (s x)/2]
-[set y position (scroller): y(p1)-vaule (s y)/2]
-Center display: position (scroller)

and volá a two player scrolling engine

example here: - http://w1.922.telia.com/~u92204154/ex/test2.zip