Question: As I was working on a login/register view for my app I came across this predicament. Login and register are two different functions; therefore they should be in two different view controllers. However, the way I designed the view makes it overly complicated to have two views because the animations for the transition between the two functions are themselves complicated. How can I build this part of the app while adhering to the MVC guidelines?

Answer: 
Apple defines a view controller as one "screenful" of information. If you think that login and signup can co-exist as the same "screenful" of information, I don't think it's worth trying to separate them. More importantly, you can separate all of the other logic to clean up this class. The networking code should be in some kind of request client, setting username/password should be in an account manager, etc.
You can easily achieve this and even go further, add some cool transition animation between the two VC by using Container View. You can easily swap two different VC in this container. I usually do that to replace UISplitView for iPad due to its problems when implementing in i.e. UITabBarController. All in all, using container view is a good way to tidy the code between two different VCs when the app requires replacing these, quite often and avoiding the solution of a s.c. Massive-View-Controller :) 

You can start with this tutorial, it includes a github repository and describes all practices with Container View.

[link for tutorial](http://www.thinkandbuild.it/working-with-custom-container-view-controllers/ "Link to tutorial")