So I am using interface builder (which I don't often use), and I have two storyboards, one for iPhone 5+ and one for iPhone 4. The iPhone 5 one works perfectly.
However, when I run the app in an iPhone 4 or 4s, one of the UITextViews shrinks in size like it has deflated. When I use constraints, I usually put the views where I want them then do
Editor -> Resolve AutoLayout Views
I agree with @Minestroni-Soup that you don't necessarily need to use two distinct storyboards. In any case, here's how I'd approach the problem. First, I'll ignore the horizontal direction because your problem happens in the vertical direction. If you have problems with the horizontal direction, post a new question.
Place constraints of fixed heights between vertically adjacent views in your layout, and also between the top view and the top border, and also between the bottom view and the bottom border. Then create a constraint that sets the height of one UITextView to be the same as the height of the other UITextView. That should solve your problem.
top border constraint to set a vertical space of some amount label 1 constraint to set a vertical space of some amount textview 1 constraint to set a vertical space of some amount label 2 constraint to set a vertical space of some amount textview 1 constraint to set a vertical space of some amount bottom border
Note that the above, alone, doesn't uniquely define the heights of the two text views but if you add a constraint that sets their heights to be the same, then all the constraints together will have a unique solution (because the labels have well-defined default heights).
I hope my explanation is clear enough.