That's what I thought, too, but a re-reading of the rules tells us this:
Double and Triple Turrets
Some spacecraft are fitted with double or triple turrets, which allow two or three weapons to be mounted in the same turret. If these weapons are different (a pulse laser, missile rack and sandcaster in the same triple turret, for example), then only one type may be used to attack an enemy in a combat round. However, if two or more weapons are of the same type, they may be fired together. One attack roll is made for all weapons being fired, but each additional weapon adds +1 per damage dice to the final damage total. (p. 158)
Right, so seemingly this would mean you have to choose one or the other, but...there are two things there that give me pause: 1) it says only one type may be used to "attack" an enemy in a combat round. Use of a sandcaster for defensive purposes is not an attack and, in fact, occurs in a separate phase of a combat round: the Reaction phase, described below. First let's consider Point Defense:
Point Defence (Gunner)
Using a turret-mounted laser (beam or pulse), a gunner can destroy incoming missiles. Note that a weapon used for point defence cannot be used to make attacks in the same combat round, and vice versa. Point Defence may only be performed against missile salvos (see page 161) as they are about to make their attack roll against a target – missiles are too small and too fast to be targeted at greater ranges. A gunner may only attempt Point Defence against once every round.
Okay, so a laser used for point defense cannot be used to attack in the same round. If you have a missile rack and a laser in a turret, then the text in "Point Defense" section seems to imply that you could fire the missile and still do point defense with the laser in the same turn. You just couldn't attack with the laser and do point defense with it in the same turn.
Fair enough. So if that's the case, let's consider sandcasters:
Disperse Sand (Gunner)
While cheap and versatile, laser weapons are easily foiled by dispersed particles, or sand as it is often called. Sandcasters are designed to create temporary defences against incoming laser attacks. Using a turret-mounted sandcaster, a gunner can attempt to block laser attacks. The gunner must succeed at a Gunner (turret) check against a laser weapon and, if successful, he will add 1D plus the Effect of his check to the ship’s armour against that laser attack only. Each Disperse Sand reaction uses one canister of sand. Sand may also be directed against incoming boarding parties. If the Gunner (turret) check is successful, each target in the boarding party suffers 8D point of damage at Personal scale.
It does describe how a sandcaster is used for defense, but also it can attack boarding parties. I might argue that using it against boarding parties would occur in the attack phase, but maybe that's splitting hairs.
The basic case is that if a missile could be fired and a laser used for defense from the same turret, couldn't a laser be used for attack and a sandcaster for defense from the same turret? Or am I getting it all twisted up and confused?