In general, I don't know that very many sites sell one day park hopper tickets. You can try looking at the all of the usually mentioned places (AAA, getawaytoday.com, ares.com, Orbtiz, way.com, your local grocery story) to see if any of them have any current specials/deals.
You could do a 1 day, 1 park, it would save $50. You don't have to use the 5 day on consecutive days, so you could do the non hopping day in the middle of the trip.
You could do a 1 day, 1 park, it would save $50. You don't have to use the 5 day on consecutive days, so you could do the non hopping day in the middle of the trip.
Agree with SMD- if you have 6 days, you probably aren't going to be hopping EVERY day...I would do something like
Day 1-4 use park hoppers to see both parks and decide which one you want to spend the whole day in
Day 5- use 1 day 1 park ticket to spend the day in DL or DCA
Day 6- use the last day of the park hopper to do whatever you want to re-ride or see again in both parks!