I wouldn't recommend two weapons that go about doing the same thing. I'd recommend a larger weapon if you have the strength, or maybe a bow for strategic purposes. A black knight sword (normal) is ALWAYS a good weapon to keep closeby in your inventory (again, if you have the stats to use it). If you can get your hands on a BKS, I'd say use that alongside your BSS.

If I may ask, what is your build. If I knew I'd be able to recommend weapons you can actually use.

My best character (the one that beat Gwyn, and the dynamic duo, Ornstein and Smough, without a shield on ng+) used a balder swag sword and murakumo. This combination gave me a pretty well-rounded melee game, and it worked beautifully in pvp as well.