How to begin next segue before first finishes

Question

There a 3 view controllers in this problem.

I'm using storyboards, segues and UIViewControllerAnimatedTransitioning with the navigation controller to customise the transitions.

I use a custom animation from A -> B, then when that finishes, B -> C animation runs (which is presenting a login form).

What I want is B -> C to start running before A -> B finishes. B -> C runs by calling performSegue in viewWillAppear of B.

I'm sure a UIViewControllerTransitionCoordinator is going to be part of this solution, but I'm just not sure where to attach it in the process.

So what I need to know is, how to begin one segue before the previous one has finished.