I am not actually sure if they will back story the two of them any time soon. Certainly not outside of what is going on with the main plot. It may be that all of these bad things will come back into the story. I am imagining some sort of portal between the two worlds being open and some nasty stuff getting into Storybrooke like the giant so maybe Mulan and Aurora will get back then. I don’t really know. This season has been all over the place, very disjointed.
Why would they want to communicate with Mulan and Aurora again? I do think that the wraith is going to come back into the story since Aurora is carrying around the medallion and i think they will save Phillip. Maybe that is where they are being lead. Back to the place they came through at the castle. They could then resolve both plot points at the same moment. Though i do think that Hooks ship comes into greater play so who knows.