The turn-around takes place at the half-way point of the journey. This means that for the entire second half of the journey, the engine is pointed at the oncoming dust.

Engines used in such long journeys will likely emit a thin stream of particles at extreme velocities. This stream is not wide enough or thick enough to stop space dust from hitting the engine. An unshielded forward-facing engine will get destroyed. So we must shield the engine.

Instead of having two shields and turning the ship around, just turn the engine around. Make the engine emit streams forward from the sides of the ship.