The splitting rules say that AD can only be split if both targets are in arc. Since boresight is a (one-dimensional) arc...
Boresight non-beam weapons can be split so long as all targets are boresighted and in range of the weapon.
Boresight beam weapons can be split so long as all targets are boresighted and within 4" of each other and in range of the weapon.
Turret non-beam weapons can be split between any targets in range, in any direction.
Turret beam weapons can be split between any targets in range, in any direction, within 4" of each other (yes that includes one target 2" in front of you, and another 2" behind you!)