Don't worry. They will find both chicks. Don't be surprised if the second one decides to leave the nest now too. Young doves tend to cling to the nest a little longer than some of the other birds. Sometimes it takes a little scare to get them to move on. Sounds like the first one was ready to go. No need to kick yourself. With young birds, as with some young adults, they sometimes just need that little extra incentive to realize they can make it.