Not to disillusion you JCR, but sometimes the order of curtain calls (as well as placement of names on the marque, and in publicity, etc.) are determined as part of the actor's contract.
And this isn't just limited to the leads. I have a friend who went into a touring production of a show in a role that had the third from last bow in the B'way production. But because there were others in the tour - although in smaller roles - that had been in the Original Broadway production - their agents got them higher placement in the curtain call bows.
Having said that... if the two roles are comparable in size and/or value to the show, having Kristen and Idina take their final bows together makes sense. For audiences who want to give a standing ovation to the show, seeing the two of them come out together would sure be a good moment to do that!

I can't wait to see WICKED. I've got a ticket for November 11th - 8th row.
Ray is the author of the Brad Frame mystery series, and two suspense novels. He is also the author of a one man play based on Ben Franklin. http://www.rayflynt.com