K.B.
Hi J.-
Start by filling in the things that are already happening "on schedule" in your house (nap times, meal times, bedtime, etc). Then I think you should think of some special activities to do with your oldest daughter during your youngest daughter's first nap - reading, art, play games --things that are hard for you to do with your oldest when your youngest is awake. Choose a time during the day (usually early morning is best to beat the heat) to get outside - go for a walk, go to a park, play in the yard. Then identify things that you can go do in the afternoon either before or after their naps--children's museum, library, run errands, etc.
By the way, I understand that the thought of managing them both at the pool can be daunting. I was scared to death when we first moved here and I had to manage my then 3 1/2 year old and 1 year old but with the help of floaties or a life jacket on the 3 year old and one of those baby inner tube type things for the baby to sit in I did ok and it was a lot of fun. Now I take my 7, 5 and 18 month old without thinking twice and my oldest 2 are not great swimmers. But that is something that you have to feel comfortable with yourself so I understand.
Good luck,
K.