Tandem Strike
Feat 6
Source Pathfinder Impossible Magic
Requirements A creature is in reach of both you and your eidolon.
You and your eidolon each attack, seamlessly targeting the same foe without interfering with each other's movements. Your eidolon makes a melee Strike, and then you make a melee Strike against the same creature. If both Strikes hit, combine their damage for the purpose of resistances and weaknesses. Both attacks count toward your multiple attack penalty, but the penalty doesn't increase until after both attacks have been made.
Traits
Anything that doesn't list another rarity trait (uncommon, rare, or unique) automatically has the common trait. This rarity indicates that an ability, item, or spell is available to all players who meet the prerequisites for it. A creature of this rarity is generally known and can be summoned with the appropriate summon spell.
SummonerThis indicates abilities from the Summoner class.
TandemActions with this trait involve you and your eidolon acting in concert. You can't use a tandem action if either you or your eidolon can't act, if you haven't Manifested your Eidolon, or if you've Manifested your Eidolon in a way where it isn't a separate entity (such as with the Meld into Eidolon feat). Tandem actions often let both you and your eidolon take separate actions, but these must be actions taken separately by you and your eidolon; you can't use a tandem action to take another tandem action.