Is there any card combination that will allow using the Ghost's dorsal turret at the same time as the main guns? I know you can do it with the Phantom docked, but it seems like secondary armament should be able to fire in the same turn as primary, even at a different target. The rules seem to prevent this by requiring either the main or secondaries fire, but not both. Did I miss something?

No.

You get one opportunity to attack with one weapon of your choice in the combat phase, unless something specifies otherwise.

You can fire both weapons in one round by equipping Gunner or Baze Malbus crew, or with a docked Phantom.

Just because it 'seems like' it should be possible doesn't mean that it is. The rules do what they say they do. No more, no less.

That's what I missed, Gunner. Thanks, that solves that issue

To be clear, the only way Gunner will do what you want here is if you attempt a turret shot, miss, then trigger Gunner to make a primary shot. Gunner (by itself) will never let you hit with both weapons in the same round.