Just did it with one cannon!
I brought a friend to help me do Galakras (10 man normal) and after first tower opened I cleared it and hopped on to cannon to clear dragons above. Second tower was only halfway-ish ready and I thought I might try it so I shot him once, he was stunned, shot him second time and he was down (actually he got stuck on top of second tower for few seconds and then went for ground).
I couldn't see anything related in patch notes so if anyone else wants to try to do what I did and confirm it was not a bug.