Weapon Research 5 should allow a non zerker to fire two weapons of the same type at the same time.

Or three if your ship has a Cerberus aug.

A skill that gives all classes the ability to multifire is not something that is missing. It simply wouldn't be possible to implement without having to spend years and years on rebalancing every single piece of gear in game.

If you wish to multifire, go Berserker or Gunner, or use a Cerberus Augmenter or even 4 pieces of Hydra gear.

And you seem to have misunderstood something about the Cerberus Augmenter. Equipping one actually gives you the ability to multifire three weapons instead of one...

Depending on AT....

Going by simple logics, I would expect that a person with Piloting 21 has Augmenter Tweaking 25.

I don't really foresee this happening anytime soon. There's already other ways to fire more than one weapon through Berserker and equipment methods. No need to have a skill for it too.