For Archenemies you have to play all the cards you want to commit prior to drawing a Titans Attack card. So, if attacking the archenemy first you have to commit how much power you are going to throw at it. Once you draw the Titans Attack card, you can't add more power, even if you still had some available. That's how I understand it anyway.
Stephen is correct. You can split the power however you see fit in the district you occupy, but more cannot be added after you commit X Power to defeating an archenemy titan.