When two players are in the scarlet biplane, they control one rotary cannon each. However, the pilot controls one gun, but this gun is fired by the second player and vice versa.

This seems to also affect the Warship.

When a gunner (second player) is present, the 2 front turrets follow their aim, but is fired by the pilot.
The 4 side turrets follow the aiming of the pilot, but cannot be fired.

To summarize:
Front weapon mounts: aimed by gunner, fired by pilot.
Side weapon mounts: aimed by pilot, cannot be fired.
Gunner seat crossbow: aimed by gunner, fired by gunner.

Although, the pack i'm currently playing is running 1.1.6, so do let me know if this has been fixed, but based I didn't see anything in the changelog to indicate so.