In my DF Felltower/Cold Fens game, a number of foes have turned up with the Teamwork perk.
In addition to what the Teamwork perk does, as written (in Martial Arts, p. 52 and Power Ups 2: Perks, p. 8), we also do the following:
- members of the formed-up unit may strike in unison with other members against a target they can all hit. Roll for each "to hit" roll before resolving defenses, damage, etc. If the defender Retreats, he is considered to still be in range of all of the strikes that hit. However, Retreat counters against all of the strikes.
Basically, this means members of the formation can strike a target together, without worrying that the foe will Retreat and end up out of range of the members.
You could reasonably say this is treated as a Dual-Weapon Attack, and if 2+ strikes hit, the defender is at -1 x (number of strikes that landed - 1) against each of them. Dual-Weapon Parry can sweep all of those strikes aside with one parry. We don't do that, because this addition is largely about making it easier to have a large group attack a single fighter without a lot of time spent on "which attack should I Retreat against?" and switching targets for the Teamwork formed-up group and so on. That the defender usually benefits a lot from this is outweighed by the way it speeds up play and makes for a better in-game feel.