That is possibly the funniest question I've seen so far. Does he want kids? Does he want two girls? Why spring it on him? Why such a short window? You do realize children do not fix a rocky relationship? Are you both prepared emotionally, mentally and financially for children? Especially twins. Have you even discussed this?
No spell can instantly make you pregnant, it's not how real magic works. I know of spells to increase fertility, or to help with pregnancy like to prevent miscarriages and ease things like nausea and labour. As a guy however, I can't say if they work or not. I can tell you no spell will get you pregnant with out some form of man plus woman equation.
I really suggest talking with him about kids before casting a fertility spell in the hopes of getting pregnant. Yes, there are guys who want to be fathers, but springing a pregnancy on a guy who isn't sure what he wants will result in problems.