To align one button to another in storyboard do this. Drag out your second button and center it by control dragging out and selecting ‘Center Horizontal’.
Then, to make your second button trail the first, control drag second to first and select ‘Vertical Spacing’.
You can then double click on the constraint to set whatever spacing you like.
To test your layout select ‘Update Frames’ from the triangle located near the bottom of the storyboard frame.
And to see all your constraints, select this well hidden slider, Hide Document Outline, near the bottom left of the storyboard screen.
And you should see a frame slide out showing you all the constraints for your current project.