The ideal placement for them is at the corner flag on the opposite side of the pitch to the referee's assistant. That way they are in a position to rule on the goal line and end line and also on sideline throw ins/ball out of bounds in their half of the pitch that may be too far away from the linesman who cannot cross half way. These are also blind spots for fouls, particularly off the ball fouls due to the distance away from the referee's assistants (other sideline or other half). Best position all round. Cant understand why FIFA never considered the positioning logically.