According to Ibrahim, there is no harm in separating the ghusl of janabah.

Hasan did not see any harm if a person, during the ritual bath of janabah, washed his head before his body or his body before his head.

Hazrat Abdullah bin Harmalah narrates that a man performed ghusl for janabah but forgot to wash his head. He told me to ask Saeed bin Al-Musayyib about the matter. I asked him, and he said that he should go and wash his head. I informed him of the issue and gave him a vessel for ablution, and he washed his head.

Hazrat Zuhri narrates that Hazrat Abu Salamah bin Abdur Rahman, when he would have lawful relations with his wife, disliked that people should come to know of it. Therefore, he would perform ghusl up to his throat, but did not like to wash his head, lest people would guess by seeing his wet hair. Then he would go to his family and say, "I have a headache," and would have a plant called khatmi brought and would wash his head with it.