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One or both of you will have to apply major breach (5948) and minor breach (2974) to create lasting effects on the target before your passive penetration. If you have the Piercing passive star in Extended Might maxed out (1400), each of you will need a little over around an additional 7678 Penetration assuming no one is using Crushing Enchant (3290 for 5 seconds).
18200 each. Now, bear in mind that (as mentioned above) that debuffs such as Major and Minor breach benefit both players and do not stack on each other. That is, if one applies major breach to the boss it helps both players and there is no benefit to both applying major breach.
This comes into play in coordinating which skills you slot. For example, both players slotting Elemental Drain is a waste. Same if one applies major breach with Elemental Drain and the other applies it with Caltrops - inefficient. That is, a coordinated duo only really wants one source of major breach and one source of minor breach between the two players.
This is one reason my main stam dps carries both a maul and a 2h sword. The maul comes out for solo stuff or when there is a need for more penetration, the 2h sword for damage when there is enough anyway.