I actually wouldn't suggest putting them in the fridge, that can also lower your hatch rate. It's best to store hatching eggs in an egg carton in a cool place (large end up), and then turn the eggs at least once a day (you can do this by simply placing a book or something under one end of the egg carton, then moving the book over to the other end to "turn" the eggs).
You won't be able to gather 60 eggs from your two girls and hatch them all together though, that is just not feasible. That would mean you'd have to gather eggs for about 30 days, and the hatch rate for eggs starts to fall drastically after 10 days. I always just gather eggs for one week and then incubate. That will give you the best hatch rate. People HAVE hatched older eggs of course, but the hatch rate is lower so you won't get as many ducklings.