2 Objects Moving in the Same Direction, which is in front and behind?

Well, that all depends on what you classify as being "in front" - if both objects truly are moving in the same direction (and you have that direction readily available as a vector; let's call it 'forwardDir') then all you need to do to determine the order of the objects is to take the dot product between 'forwardDir' and the vector from one object to the other;

 // For simplicity, let's just assume 'forward' means along the z-axis;
 Vector3 forwardDir = Vector3.forward;
 
 // Then, find the vector going from objectA to objectB;
 Vector3 delta = objectB.position - objectA.position;
 
 // objectB will be in front if the dot product is greater than 0.
 // If the dot product is equal to 0, then both objects are equal and you can just pick one (in this case we will default to objectA).
 bool objectBIsInFront = Vector3.Dot (forwardDir, delta) > 0;