Just joining this thread, so I apologise if anything I say repeats anyone else's suggestions/comments.
As far as I can tell, the DB that the system already provides shields is based upon the obstacle that it presents the attacker. It's much the same as the target being made smaller because they are standing behind a wall. They get hit less because they are more difficult to hit. This reduces the damage since it makes the attackers roll less than it would have been. Simply that.
The number of opponents are affected is based purely upon the size and it's relative position. It doesn't take into account any active defensive skills that the user may attempt with the shield. Any parry that the defender uses (probably derived from a weapon skill) serves to further increase the difficulty of the attacker to make a telling blow. In order to do that NO skill roll is required...
But... lets say that the defender has ONLY a shield to defend themselves with? They can just passively hide behind it , absorbing blows raining down on them... or if they are competent (and strong enough) attempt to make active attempts to interpose the shield, increasing it's effectiveness. That part, really, is no different to using a weapon to parry, still NO roll, just an application of parry using a shield skill.
Shields do absorb blows (until they fall apart).. but much like plate armour and sloped armour on tanks can also deflect harmlessly. *That's* what an active skill could conceivably be used for.. in that case the skill is rolled. If it succeeds then there is a derived % chance that the attack is deflected harmlessly... However, the user MUST dedicate activity specifically to do so... some GM's might also want to want the skill developed in the same sort of fashion as Two-weapon fighting.
However, the truth of the matter is, if anything hits a Shield hard enough, the concussive effects WILL affect the wielder or perhaps even unbalance them if it isn't deflected.