If one of them had the book to themselves, and the other was given a backup, that'd be one thing. If they were unconnected characters whose books were combined, title and all, because of the economy, that'd be one thing.
But they're married, and thus they're likely together a lot. They share the title. Not making them a team under those circumstances is rather a waste - occasionally giving one or the other a solo story, sure, but changing the book's format to 'two solo stories' is a waste.