I would add that the versatile soldiers shouldn't do as much damage with the specific weapons as soldiers trained with only one weapon unless the versatile soldiers spent more time in training.
Of course, in my mind, training a soldier to use a weapon would increase build time by
x. For
any weapon.
Let's say that it takes 10 turns to train a basic soldier. No fancy things, no extra training. Once you give him a sword, it would add a flat 5 turns. Now it takes 15 turns.
Teach him to use javelins (to throw in the initial fight, like the Romans did) and it adds another flat 5 turns. It's now 20 turns.
Why flat and not percentage? Because there's going to be many more kinds of training in Elemental, gathered from dev. journals by Frogboy. If Advance Combat Training added another 20 turns to the base 10 turns of training, and then weapons each added 50%, the cost for doing so would grow exponentially, discouraging the use of advancely trained soldiers with multiple weapons - the exact kind I'd be most likely to want to train in several weapons.