All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If you intended to perform two ‘Umrahs, you can shorten your hair during the first ‘Umrah so that there would remain some hair to shorten or shave during the next ‘Umrah. This is because the obligation is fulfilled by shortening despite the fact that shaving the hair is better. The Prophet, sallallaahu ‘alayhi wa sallam, said: "‘O Allaah! Have mercy upon those who shave their hair.’ They (the Companions) said, ‘O Messenger of Allaah! And those who shorten their hair?’ The Prophet, sallallaahu ‘alayhi wa sallam, said in the third time, ‘And those who shorten their hair.’" [Al-Bukhari and Muslim]
If you shaved your hair, you should wait until your hair grows then perform the second ‘Umrah. If you performed ‘Umrah before your hair grows, this would be permissible.
Ibn Qudaamah said in Al-Mughni, "‘Ali, May Allaah Be Pleased with him, said that it is permissible to perform ‘Umrah once a month." Anas used to perform ‘Umrah whenever his hair grew enough to be shaved. This was narrated by Ash-Shaafi‘i in his Musnad. ‘Ikrimah said that one can perform ‘Umrah if he had hair that could be shaved by the razor. ‘Ataa’ said that one may perform ‘Umrah twice every month if he wanted. Ahmad said, "If a man performed ‘Umrah, he should shave or shorten his hair and the hair can be shaved every ten days."