Does the gov't need a second warrant to open a letter-in-a-letter?
Interesting question; I'm guessing no, unless the interior letter contained obviously privileged material (eg 'Dear Father, I hope you are well; enclosed is a letter I received from your lawyer after you departed, which I forward to you unopened. Your Loving Son.'), in which case a warrant would probably not issue for the contents.
None of which is responsive to the question asked above, which was about a letter that was lawfully opened pursuant to a warrant, but which contained a second, sealed letter.
Interesting question; I'm guessing no, unless the interior letter contained obviously privileged material (eg 'Dear Father, I hope you are well; enclosed is a letter I received from your lawyer after you departed, which I forward to you unopened. Your Loving Son.'), in which case a warrant would probably not issue for the contents.