If a man emits semen twice or more, at separate times, then he does one ghusl, that is sufficient. The Prophet used to go around to all his wives, then he would do one ghusl, as Muslim (309) narrated from Anas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) used to go around to all his wives and do one ghusl. 

The fuqaha’ have stated that if there are several reasons that necessitate ghusl, such as having intercourse several times, or if the two circumcised parts meet and ejaculation takes place, or if a woman is both junub and menstruates, then one ghusl is sufficient, according to scholarly consensus. 

Al-Nawawi (may Allaah have mercy on him) said: If a person breaks wudoo’ several times, then one wudoo’ is sufficient, according to scholarly consensus. Similarly if a person becomes junub several times, by having intercourse with one wife or with several wives, or by having an erotic dream, or all of these, one ghusl is sufficient according to scholarly consensus. Among those who narrated that there was consensus on this point was Abu Muhammad ibn Hazm, and Allaah knows best. 

End quote from al-Majmoo’ (1/487). 

And Allaah knows best.