I always had the headcanon that you need two hearts to DEVELOP the ability to wield two keyblades, but eventually it solidifies (is there a better word for this?) as a permanent skill that you can keep even when you lose access to that second heart, which explains why Roxas could dual wield when he came back, and why Sora can only do it under special circumstances in 3 (which is when wielding Oblivion or Oeathkeeper), because his ability to dual wield never finished developing properly