After doing some testing myself the best damage output for dual swords seems to be Master Swordsman 4 piece, Tatanashi 3 piece, Kingo's 2 piece with the chest piece being a free slot for any light armor set bonus. Its actually about a 28% damage increase from the Kingo's and Master swordsman's build I was using prior.
Set piece breakdown:
Head: Master Swordsman
Chest: Iga Ninja
Arms: Tatanashi
Legs: Tatanashi
Feet: Master Swordsman
Accessory 1: Yakasani Magatama w/ Yokai close combat damage , + Fire damage, Living Weapon attack enhancement, Unlimited Onmyo
Accessory 2: Magatama w/ Yokai close combat damage, +Fire Damage, Unlimited Ninjutsu, Defense scaling with Spirit, Onmyo Duration Rank C for Extraction talisman
Primary Wep: Master Swordsman + Fire
Secondary: Kingo
Primary Ranged: Bellowing Cannon (for extra + fire damage)
Secondary Ranged: Saika's
Elixir
Quick Change Scroll
Tiger Running Scroll
Extraction Talisman
Weakness Talisman
Power Pill
Makibashi (ninja slow skill) / Suppa Scroll / Shadowround Talisman
Sacred Water / Keikai Talisman (when going against bosses)
Guardian Spirit:
Tengen Kujaku 1st / Kara Jishi 2nd (You can Swap Tengen for Suzaku for even more fire dot damage but I prefer the bigger on hit damage boost from tengen)
I also have a Lightning Master swordsman dual kat but I dont yet have the same amount of Lightning damage to stack for it and it only has A skill scaling so its damage is substantially less but it does have a roll for additional damage from Sign of the cross so that particular skill hits really hard. Provided you have gear more suited for it or prefer lightning you might want to go with Kara Jishi as your primary for the added lightning damage with Tengen as secondary for the extended elemental duration. It also has the added benefit of not being reliant on extraction talisman for the damage boost so that frees up an item hotkey for another Onmyo magic or ninjutsu item. I actually prefer the elemental effect so I'm working on getting the necessary accessories for that build now.
It also has the advantage of being incredibly high toughness but the catch is you'll need high investment in Str/Stam to maintain B agil. You'll want Agil damage bonus and Familiarity damage bonus on your wep and ranged wep. Consecutive attacks is crazy powerful but can be tricky to build up so I prefer the flat boost of the former two. I'm currently using a Fire Master Swordsmans Dual Kat with 22 fire damage, +damage from fire on wep 44%, A agil damage bonus, AA Skill Scaling and 25 close combat life recovery. Base damage in tootltip with full familiarity at +17 is 2390. My current chest piece is the Iga Ninja set fr the ninja skill replenish after LW. It has Reduced elemental damage, Life from amrita absorption, attack (transfer), + toughness (to put me just over a toughness threshold with my title perks) and unlimited ninjutsu. as stated prior this build was a massive damge boos from my Kingo'/Master Swordsman set. I also tested Warrior of the East + Master Swordsman and Warrior of the West + Master swordsman. Warrior of the east was actually also a minor improvement over kingo's and Warrior of the west was a net damage loss over the kingo's build.