Glenn Reynolds examines the question here.
Perhaps there is a distinction between "The Executive officer acting in accordance with the dictates of Executive function" AND "The Executive officer acting in accordance with the dictates of Legislative function."
I'm much more interested in the question of why it was Constitutional for the electors of Texas to vote for both Bush and Cheney, given that both of them resided in Texas.
So Truman was forced to make a momentous decision-- one that was very controversial and, even if justified, took thousands of lives-- in a very short timeframe between April and July 1945.
So, insofar as the office of Vice President has an executive character because he can temporarily assume Presidential duties or succeed to the Presidency, so too have the offices of Speaker of the House and President Pro Tempore of the Senate.
No, because they are not exercising executive power in the course of a normal day,
Putting aside the strong argument that that's unconstitutional
As anybody who watched The West Wing knows, they have to step down from Congress in order to assume the presidency, even temporarily.
What strong argument? The President Pro Tempore and the Speaker of the House were placed in the line of succession under the very first Presidential Succession Act (1792). Certainly Madison didn't like it, but as he was a partisan of the Jefferson camp at a time when Jefferson was Secretary of State, his opinion needs to be taken with a grain of salt. Washington, who was just as much at the Convention, clearly did not believe it was unconstitutional.