Logically speaking, Roxas had been recently born when 358/2 days begins, long before kh2, Sora has always used one keyblade, and Roxas being his Nobody would naturally adapt to some mannerisms without his realization, since he lacks Sora's memories. I think as the game progresses and Roxas steadily becomes more powerful, he will develop his signature dual wielding style.
Or maybe the 14th member gives him the power when she leaves the Organization, who really knows...
Or maybe the 14th member gives him the power when she leaves the Organization, who really knows...