If more than one magician protects a target with Counterspelling,
handle it as teamwork.
That's SR4 though, and enough changed between SR4 and SR5 that I wouldn't simply copy this rule. For one, in SR4 it wasn't as easily depleted as it is now.
Personally I'd say there's nothing wrong with just add it up (the way Aaron said). If there's 2 mages with 4 counterspelling each, and they both use their entire pool against a single spell, it's going to be though for the opponent. But after that the pool is depleted.
Making it into a teamwork test (add one as normal, and roll the other as a seperate pool with skill rating alone, and add hits?) would make single magicians less of a joke against multiple-caster teams, so I get where it's coming from. GM's don't like if players become nigh-invulnerable to magic because they have a counterspelling pool of 10+. But the teamwork-aspect would make it quite weird.