I would imagine you need to install a second cannon. I think these days dogfights are fairly unlikely, jet fighters are more likely to shoot each others beyond the horizon with the advantage to the side with the missiles with the longest range. A second cannon is a lot of weight for a very remote use case.
But when you're in a disadvantage with conventional guns... if you had guns that could shoot from any direction - wouldn't you not be in a disadvantage anymore?
