Sorry to disagree, but I think splitting the narration between a first person and a third person would be a mistake. I have seen it done, but the only time I tried to do it myself my agent was barely polite about it. It's the only thing I ever wrote that she didn't like.
I have no problem with two third-person narratives, as long as it's always clear who we're with - and there's a clear understanding of what each character could and couldn't know at any given point.