According to Article II, section 1, clause 6 of the US Constitution, in the case of death, removal, resignation, or inability to discharge the powers of the office, both of the President and the Vice President, Congress may by law provide who will act as President (and it currently does in the Presidential Succession Act, with the Speaker of the House as first in line).
I have a question about how this would work in a case where both the President and Vice President are incapacitated (i.e. alive but unable to perform any actions).
The 25th Amendment, sections 3 and 4, provide the two ways in which the President can be determined to be unable to discharge the powers of his office. Under section 3, the President can declare himself unable to discharge the powers of the officer for a period of time, which he can't do when incapacitated.
Under section 4, either the Vice President and a majority of the cabinet, or the Vice President and another body that Congress may by law provide, can declare the President to be unable to discharge the duties of his office. Since the Vice President is also incapacitated, this is not possible either. So in this case is there no way to declare the President (and the Vice President) unable to discharge their duties to have someone further down the line of succession act as President? (I guess one alternative would be for Congress to impeach both the President and Vice President, which would immediately make the next in line the Acting President, although it seems unreasonable to impeach them if they did not do anything wrong.)