I'd stat each sword out individually.
For the purposes of stunts, he might have to decide which is the 'main' weapon. Although, I don't think it matters much.
This way, if he looses one sword, he can mark off the powers he loses.
Alternately, you could say he gets no powers unless he has the pair.
Or you could stat them each with their own powers and attach a few to a limitation that requires both swords. (as Haru has pointed out.)
-5 Iop swords +2 (obvious) Total refresh cost: -3
-2 Inhuman swordsmanship
+1 limitation (only applies when both swords are wielded together)
Sword (Snip) -2
-2 Inhuman Speed
Sword (Cleave) -2
-2 Inhuman Strength