How switch between screen inside screen?
Ah. screen-in-screen... very Inception. We need to go deeper...
That being said, you can send a ctrl-A to a nested screen by doing Ctrl-AA.
Ctrl+A, 9 or Ctrl+A, " (that's a double-quote) and then select it from a list.
As mentioned by rocka84 but for the second command you don't need to press Ctrl again. Like for creating a new screen in nested terminal you need to do like this Ctrl+a, a, c.