Different people grieve in different ways, and some take longer than other. I have a friend who lost a beloved cat a couple of years ago (she had shared 18 years with this cat) and still isn't ready to get another one, even though I've recommended to her several times that a new kitten might be just the thing to keep her remaining cat company and bring some fun to both their lives.
To me, a month seems awfully soon. But you should do what feels right to you. Just be careful to avoid the trap of trying to emotionally replace Harvey and Ralph. Your new bunny is a new individual with her own personality and habits. She will be unique and special in her own way.