Forgot about line of site rule.
If no opposing squadron has LOS due to ship obstruction, then they are not engaged and could attack which ever target freely.
Or obstacles. I often find enemies forget that and maneuver ships near obstacles. Just fly the squadrons into the obstacles, and *taddah* - no more being worried about getting 'engaged'. (Granted, depending on positioning, their own shots may be obstructed - which is pretty bad)