How do I control a screen session inside a screen session?
ctrl-a a is the equivalent of sending ctrl-a to the child screen, so ctrl-a a d should disconnect from the first nested session. If you have them nested deeper, ctrl-a a a d will disconnect the second nested session, and keep adding additional as as needed. If you're nesting deeper than 1 or 2 levels, though, don't ask for sympathy when your head explodes from trying to keep straight which session you're sending commands to...