For lasers and partical accelerators (maybe fusion guns as well), you could "pipe" (via mirrors or magnetic channels) the beams to whichever side of the sphere was engaged (depending on whether you are the one chasing or the one being chased). For missiles, you could have a central magazine that feeds to launch tubes both fore and aft, at least if you are using relatively small missiles.

I'm not sure if such partially-duplicated arrangements would be more or less efficient (in terms of required volume and power) than dedicated directional mountings. But it may work out to be a wash. Given the ranges at which combat between *capital* ships (as opposed to cinematic fighters) take place, you aren't very likely to simultaneously engage targets outside a fairly restricted arc. So a certain percentage (as much as half) of your weapons aren't going to be usable in a given action. If you instead mount a single larger weapon but feed the output in opposite directions, you might come out with an effectively more powerful armaments array . . . at least for most battles.