Wouldn't you need one on each side of the goal.
Then you have to decide who gives the decisions, imagine one linesman flags for a free kick outside the box as they see the incident, the other signals for a penalty, the ref is unsure! Which official does the ref go with?
In my opinion technology should only be used for decisions that are not open to interpretation i.e. has the ball crossed the line. This should only be used once it is 100% accurate. The addition of more officials will only make things more confusing.