At the end of KH where Riku emerges in front of Ansem to protect Kairi and company, Riku's heart must have been really strong to stall Ansem(supreme ruler of darkness). Did he do this with a sudden spark of light from his heart that was overcome with darkness. Err...I don't know where I'm getting at, but I agree with you cloud@omnislash; Riku's purified self probably does contain darkness, and he could still have the power to control the heartless. I don't necessarily know why he'd want to control them again, but as cloud said before, it could have been a diversion so DWU and BU(has to be Riku) could exchange keyblades.
Hey, you're right! The heartless could've been afraidof the presense of TWO keyblades(which is why they hesitated), but they were all charging at DWU in both movies despite the fact that he had two keyblades.