For a temporary solution, make a second fire group with the multicannons swapped for the KWS (since you'll likely be scanning while taking down shields, if you're using multicannons for shields I pity you for your ammo costs

), and under controls you can bind shield cells to a seperate button.
I agree though, i want to be able to have more than two types of weapons and fire them all independantly. At the moment I'm running four low heat gimballed beam lasers and two class 2 railguns on my Asp, but I'd love to have two multicannons, two lasers and two railguns and be able to fire then seperately with the same fire group. I can only imagine it gets worse for those with an anaconda.