if one boy and girl's marriage are fixed another girl can pray for marriage with the same boy. please give the answer in the light of islam.

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 is His slave and Messenger.

If you mean that a woman supplicates for this marriage not to take place or for divorce to happen if the marriage does take place so that the man marries her, then this is not permissible as this is transgression and oppression. However, if you mean that she supplicates Allaah to make her the second wife for this man then this is permissible, Allaah Willing.   

However, it is more appropriate for a person to ask Allaah to choose for him what is best. One knows what he loves and desires but he/she does not know what is beneficial for him/her in this life and the Hereafter. Therefore, it is better for a person to surrender his affairs to Allaah.