If you get to the MK and 4pm and make a gameplan, one night is enough. The candy isn't all that great, it's the same candy at all the locations. It's fun, but you can only haul around so much candy. And towards the end of the night, there are no candy lines, you breeze right through. The park empties out after the fireworks and the first parade, if meeting the 7D, HM, parade, and fireworks are all you want to do, you will have plenty of time. I'd save your money for a future trip. At at least $150 per ticket per person, if there are 4 of you, that $600 could go a long way towards another trip - to attend
MVMCP. It's a great party, but not necessary to go twice considering your wants.