Could be easy to program.
Example:
Place an invisible area-sensor block (area activation block)
Slave that invisible block to multiple weapon computers or barrels selected as the output ports.
When fired, visible beams are drawn from the emitter barrels, converging on the invisible block, then the real "beam" is fired from the invisible block in the direction of the primary weapon computer. (I guess it could be aim-able from the invisible block like a normal weapon, but that might look funny.)
Maybe nerf the range by 10% or so and leave the invisible block unshielded when slaved, so that the practice of using this method to extend the range of the weapon is not so OP.
Edit: This idea could be extended to allow builders to create in-ship beam effects.
Illustration: Just ignore the foreground action and look at those sexy beams in the background.