Okay, so abilities I'm adding.
Back to Back: At 3rd Level, two swordsmen learn to coordinate their movements such that they can operate in close quarters without getting in each other's way. Partners can occupy the same five-foot square, and are immune to flanking when they are doing so. In addition, if one partner fights defensively, or takes a Full Defense action, both partners receive the AC bonus.
Set-Up: At 4th level a pair of Swordsmen learn to, even as they attack, set up their partner for a second attack. Being struck in melee combat by one partner provokes an attack of opportunity from the other once per round. This ability is not triggered by Attacks of Opportunity.

Retaliatory Defense: At 5th level a synchronized swordsman is able to defend against attacks in such a way as to make an opening for their partner. Attacking one partner in melee provokes an attack of opportunity from the other.
Avenge the Fallen
: At 7th level, the bond between the partners as grown so strong, that killing one only enrages and strengthens the other. If one partner dies, the other Rages as a barbarian of level equal to their character level.
And this next one may be overpowered
Twinned Blades: At tenth level the partners have reached a level of synchronization that makes them nigh-unstoppable in close-quarters. When they are occupying the same five-foot space, or if they are flanking the same foe, the partners may make a special full-attack action once per encounter. In this action, they both make full-attacks, alternating individual strikes ( fPartner A Attacks at his full AB, then Partner B attacks at His full AB, then partner A does his second attack, ect ect). If any attack deals at least two damage to a foe, then subsequent attacks to the same foe receive a +2 bonus to attack and damage. These bonuses stack for every attack that deals at least damage. (So, if Partner A's attack hits, then partner B's attack receives a +2 bonus. If that hits as well, Partner A's second attack receives a +4 bonus). They can only perform this