Hi Amanda,
You picked two incredible Walt Disney World Resorts to stay at! At the time that I am writing this answer you can book Advanced Dining Reservations 180 days in advance of the first day of your trip. Since you are staying a two separate Walt Disney Resorts, they are treated as two separate vacations as far as Advanced Dining Reservations are concerned. So you will have two different dates that you can start to make your Advanced Dining Reservations online or by phone. You will have to book only the restaurants that you plan on eating at on your first leg of the trip first and then all of the ones on the second part of your trip a few days later.
I used a handy website called the Planning Strategy Calculator for Disney Reservations located at www.pscalculator.net/pscalc.php to calculate when you should start making your Advanced Dining Reservations for each trip.
For your Polynesian stay on August 21st to the 24th Your 180 day time starts on 2/22/2010
For your Bay Lake Towers stay on August 24th to the 29th Your 180 day time starts on 2/25/2010
Also keep in mind that you must use up all of your credits from the first part o your trip by midnight on the day you check out of the Polynesian and you will not get your new dining credits until you check into Bay Lake Towers.
Go to
www.disneyparks.com look under Walt Disney World then Book and then Dining Reservations to make Advanced Dining Reservations online or call 1-(407) WDW-DINE or 1-(407) 939-3463 to make them by phone.
I hope all of your dreams come true on your next trip to Walt Disney World!
Keep Dreaming Big!
Marc Lorenzo