IIRC you can't swap the heads around. The sensors on the toe arms have to be able to physically/line of sight see the one on the opposite sensor. It doesn't need laser precision, they just need to be able to see each other.
And the sensor at the back of the LF head needs to be able to see the one in the front of LR head. And ditto the above if it has rear toe arms, which are pretty rare IIRC.
If the car you are aligning has more than about 2.5 degrees of negative camber in the front, the toe sensors are not able to see each other. Actually the sensor is fine, the plastic window in the head is not large enough. If you take it apart, you can see a place where you can enlarge it. Not sure why they don't do this from the factory.
HTH
DaveW