You could also try some interactive play with the one thats not so scared, where she can see her enjoying herself. Wait until the house is quiet and theres just you and use a rod toy to play with the brave kitten. Babies can not resist playing. She may not come out immediately, but it will grab her attention. When she does come out, carry on playing wiggling the toy near her occasionally, but don't look at her or speak to her. She will come round eventually. Like Mattyh said patience is the key
