Well, got to find something to do with them youngin's!
The berserkers who survive their first few battles and learn to fight with a level head live long enough to become veterans.
I also think that part of the Berzerker mystique was to strike fear in the enemy.
"Okay Jerry, here is the plan. You wear this bear skin shirt, and scream really loud like a crazy man. Freak them out, while me and Bob come up from behind them".
Also strength to kill a man in a single blow, thick layer of fat and muscle below the skin that they can keep going even with some heavy wounds and speed to outrun any man that tried to flee.
Certainly helps. And the berserks generally were not exactly runts. All right, we're pretty far afield from the original topic now, but:
The berserks: what was wrong with them?
... English summary of an article from a Norwegian medical journal. (full article, in Norwegian)
Well, shield DID fall out of use more and more, over time it became far more common to use a sword and dagger or sword and short sword or similar combination.
The best preserved books about fencing instructions, from 16th century IIRC, deals only with sword and "2nd weapon", commonly a "main-gauche" en.wikipedia.org/wiki/Parrying_dagger
while shields are barely mentioned(ie it had fallen out of use just about completely by this time).
So the fact is, it WAS standard training. Shield was used in organised formations of troops, 2nd weapon was used as a personal way of fighting more effectively(and without needing to run around with a 5-10kg shield everywhere), but eventually shields fell out of use in organised troops as well because they were unwieldy, couldnt be used with pikes(which became the weapon of choice(read about Spanish tercio if you want more))...
Except it fell out of use exactly because it wasnt superior(apart from running around with 1kg instead of 5-10kg extra of course).
Two axes should give a penalty instead, its an almost impossible combination to use effectively(the head holds the center of gravity and is also the only point which you can parry an edged weapon with).
Sword&dagger style became popularised during renessaince yes, but it was already in widespread use by that time.
And late renessaince actually went further towards the single light and fast weapon rather than sword&dagger.
Oh it might be more effective, but you should probably do it by an offense bonus and a smaller penalty to defense. One way or another. That should keep it reasonably realistic with at least some styles.
Since 2-weapon fighting basically only works when you're not fighting as part of an group, it might be an idea to allow the fighter to count as in a trained group with himself - giving a bonus to either offensive or defensive according to his weaponskill. This would be instead of any bonus from the weapon itself - and would be instead of having a bonus from any allies. Might be too good though... So maybe a bonus equal to the secondary weaponskill - 2?
Well, fighting with two weapons is extremely diffucult to learn. You have to dedicate your live to it to match (!) a warrier with a single weapon. And only if you were extremely skilled you might surpass the others. This takes into account that the opponent will be unused to any way of two weapon fighting in Mystic Europe.
Just to give you an example: In today's Kendo (Japanese sword fight and kind of national sport in Japan practiced by more than 1 million Japanese and many people from other countries) the classical way is to fight with a single two handed sword. However, you are allowed to fight with a sword and a second, smaller one. This style is adopted by some fighters who parctice a lot (most likely professionals). Nevertheless, none of them has ever won the world champion ship or All-Japanese-Championship. Here the single sword fighters prevail.
There is is one exception: Musashi who fought and invented the two-sword-style and is regarded the best sword master who lived being revered to as a sword-saint. If you want to play such a character (tempting in a mystic setting) you should not only dedicate the character's life to it (spending at least 50-75% of all XP on Sword and/or Two-handed fight ability) but give him the unique physical stats needed: High Strength, Dexterity, Quickness and at least one major and three minor virtues enabling to fight this way.
No, not THAT bad. More difficult certainly.
Yes, i know. However, the double-style isnt completely accepted within the sport from what i have heard so you probably dont get the best suited persons using it.
While its harder to dual-wield yourself, try facing an opponent dual-wielding even halfdecently, its VERY much more difficult to fight against than someone using "regular" single-wielding style. Cant parry 2 places at once, and using shield really isnt much good either because its so much slower than using 2 weapons that its easy for the dual wielder to outmaneuver weapon+shield user.
So its not just bad stuff for the dual-wielder.
Waaaay too many virtue points for something as really minor like that. Even the best Kensai ever would still be a minor person in any Ars magica setting.
I would suggest something like already mentioned, a "two-weapon fighting style" minor virtue and connected ability limiting the bonus in a fight and limited by the single weapon ability itself.
Making it a "cool" thing for an occasional grog or companion.
Exactly how to handle the mechanics, id rather leave to each SG instead.
The shield is much better for the formation fighters with spear and short sword. IT is also very important for the knights with lance as a second weaon is not going to parry a lance so much more usesful in mounted combat.
It is true though that pike units wouldn't use the spear and many individual fighters and fights might go two weapon. IT is a cultural thing.
Then again, I am reminded of my SCA friends that told me about fighting Florentine Henway.
(Is anyone willing to admit this has gone off-topic yet?)
There is no such thing as off topic. 
2 weapons entered the area not because it was superior, but because they could be carried around effectively without getting unwanted attention. Cloaks became illegal in Spain for a reason, after all. it is a duelling weapon. In the really bad Alatriste production (that totally messes the original storyline) you can see a fair show of how this was used.
Far from superior to the shield. Fairly different fighting style based more on the "dance like a butterfly sting like a bee" than the "smash brains all around the area" concept of previous combat periods. Good for duelling, fairly bad for melee against organized opposition. It certainly favours offensive mentalities more than defensive ones; the bonus is not on the later stances when you are using only 30 cm of steel to parry (at most) instread of a shield
Still, it wasn't practiced in medieval europe. It can be practiced in MYTHIC europe, though. I will not favor it, but who am I to tell you you can't do it in your saga? 
Cheers,
Xavi
You can? You can use two weapons and parry two stikes at once? I am impressed! Most people have a problem: They do have two hand but only one mind.
In the end it boils sown to: Who manages to lure the other into a bad position and is able to recognize this in a split second before he can correct his mistake.
In fencing, I found dueling with two sabbers to be more difficult than it was worth. Mind you I only took that classs for a single year, so I am no expert. But it seemed more of a hinderance than a help. Showy though, and that what really counts 
So you asked how a sword expert should be like. Well, I would imagine him along the line as shown in the movie "Rob Roy": When Robert was attacked in the tavern, he simply dodged the attacking guys stoke and killed him in the same move. Combining brawl and single weapon seems more suitable to me. This way you may use punches and trip you opponent when you are close.
I always assumed that hand weapon (and 2 handed weapon) combat included a fair bit of brawl manoeuvres like tripping, kicking spitting and elbow punches. Still, the manoeuvres are performed with the idea that you have a sword, axe or mace in one of your hands. That changes the kind of maneovres and dodges that you perform if you only have a knife in your hand.
So to me keeping them 2 separate abilities is perfectly fine.