Do you mean all Representatives and Senators would be elected at once every four years? And would the Senators still be elected by the state legislatures rather than the people? Those factors would have some effect here. If the state legislatures still decide who's in the Senate, then the election of state legislators will become (even) more tied to national politics. The outcome generally, however, would be that congressional elections become far more important. A congressional majority ensures that your party gets to decide who becomes President.
Since the President is installed by Congress, I assume he'll be subject to a possible vote of no confidence, and that Congress can basically dismiss him and his ministers and appoint a new President at any time? That would make sense in such a system. The effect would be to make the executive far less independently powerful. Congressional primacy all the way. Incidentally, would the President (like the rime Minister in Britain) be elected from within Congress, and would he retain his seat there? Or would Congress be obligated to choose someone who is not in Congress?
This is relevant, because we may probably assume the USA still uses electoral districts. This means that if you were to put a member of Congress in the Oval Office... he'd kind of have to retain his seat. Because if not... who replaced him? The one who get the second-highest number of votes in his district? That's probably his opponent, so he'd be giving the opposition an extra vote. For this reason, if you use a district system and Congress appoints the executive, then you'll either need to exclude all members of Congress from consideration, or you'll need to put it in the law that members of the executive retain their seats in Congress if they're already Congressmen.
End result: the USA would look a lot more like Britain, in the way its political system is organised.