Is it necessary to perform one ghusl with the intention of both menstruation and janabah, and what is the ruling on missed prayers for someone who has purified from menstruation and waited two days to confirm, then semen was discharged from her?

The questioner erred by delaying her post-menstrual bath for two days. She must make up the prayers of these two days because they are a debt upon her. Her ghusl (ritual bath) with the intention of both menses and janabah (major ritual impurity) is sufficient and removes both impurities.