Just mark the first one(I used the "parent" variable for this), and make a global variable wich stores the original ball's angle.
+ System: Start of layout
-> System: Create object Ball on layer 1 at (50, 400)
+ System: For each Ball
+ Ball: Value 'parent' Equal to 1
-> System: Set global variable 'angle' to Ball.Angle
+ System: For each Ball
+ Ball: Value 'parent' Not equal to 1
-> Ball: Set angle of motion to global('angle')