If he becomes junub twice, one ghusl is sufficient
Question

What is the ruling on doing ghusl (full ablution) if a person ejaculates twice in a row at separate times, knowing that he did not do ghusl the first time. Should he do ghusl twice or is one ghusl sufficient for both?.
Praise be to Allah.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 Allah be pleased with him) that the Prophet (peace and blessings of Allah 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 Allah 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 Allah knows best. 
End quote from al-Majmoo’ (1/487). 
And Allah knows best.