A shield synchroniser allows the placing of two shields on one facing.
Usually the interactions between the LDA's at such short range causes a huge power spike and the smaller shield overloads and collapses. With the synchroniser in place the two shields can be tuned to not interfere with each other.
A synchroniser is added to the main shield unit on a facing and then a smaller shield (with a light shield lower rating than the main shield) can be added and tuned to match the main shield. This allows a ship with enough power to have two shields on one facing.