Under "How Many Shots" page 137, it says each model may only shoot once. Unfortunately I don't see any rules that would make the exception that they could shoot both the cannon and the gun.
However, chariot says that the crew may attack separately. So I think you get the engineer's weapon, plus either the cannon or the gun.
It's kind of a bummer because I remember shooting both in 7th edition. I think you could also shoot the steam gun in combat in 7th edition. That's a change that would make the Steam Tank more worth taking.