I do not believe it is possible for a melee combatant in Ars to wound more than one person in a turn. He can ATTACK more than one person; this happens when he is attacking a Trained or Untrained Group. So if, for example, five Teutonic knights team up to fight one PC, he can attack the group, but any wound he inflicts will only be inflicted on the Vanguard of that group.
If you decide to add rules for attacking more than once, take special care with the rules for defending another person. In the rules, one individual can protect another individual, taking all wounds and forcing all enemies to attack the defender. This isn't especially realistic, but it is an artifact of the Shield Grog/Magus relationship baked into Ars Magica. If attackers can attack multiple people, you will need to decide if this allows them to attack both the defender and his magus at the same time. If it does, your shield grogs are less useful, and magi are in more danger.
Physical combat in Ars Magca is very abstract. Rolling for an attack should not (I think) be equated with a single actual attack.
That said, as I understand the combat rules, it s not possible to damage more than one opponent at a time (without magic).
With the Ready Missiles option in the Lords of Men Supplement, a bowman could fire his bow (arrow nocked) as a fast action, and then nock another arrow and fire as a standard action, allowing two attacks in a turn. In terms of melee combat though, I can't recall a way to do this.
Is not in my mind to create a new rule for that, the rules are the rules and should not be modified, but the rules about engaging/desengaging in the lord's of men book are so abstract and one player may think that he can hit more than one enemy in one turn, in fact is so strange that a player can defend him from "n" enemies and only can hit one!! smells strange hahaha...
... I don't know if you know that Ars Magica (and by extension, its creator company Atlas and most of its players) is probably the game (/company/player base) that is the most supportive of interpreting, adding to, and modifying the rules of the game of all currently available RPGs. There's probably competition, but I've yet to see it. I personally recommend treating the rules as mold-able, good in their structure but also free to be changed if you've out thought into it and decided it doesn't currently work right for your troupe. Just food for thought.
Probably you are right, but if i start to create custom rules, is easy break the balance of the game. We are playing ars magica and the magic (and it rules) are amazing and I think the melee rules was left unfinished, is my mind of course... but that's not is the meening of the post.
Working outside RAW - A character might be able to strike at two targets or strike twice if they are using two weapons or/and if you agree that the combat round is an abstraction of combat (as said above). Wielding two weapons is certainly not common in the literature of the time, but if you like that kind of style then go for it.
I'm a fan of using Ars for high fantasy as well as gritty medieval settings, so I'd allow it as SG in the right stories. I have draft rules for it too.