He probably knew they'd get healed?
Plus it was only way to get P back?
I haven't watched those episode so saying from what I read in forum
Aj, when he gave the Khanjar, insisted that it has to remain inside Dee. That was needed to ensure that Pi didn't get her body back. But then he sees Dri dying and was shocked enough to exclaim something in the sense that he did not mean her to die (from his words I assumed he didn't think killing Dee will end Dri's life too). He says he will not let it happen and then pulls the knife off Dee giving her a chance to live.
That was my interpretation of the scene