Plz HELP!!! I need answers!!!?

Im getting two rabbits this friday. one girl and one boy from the same litter. I can only get one cage. they will be 7 and a half weeks old. ill will get them fixed @5 months but i dont want a pregnant female before that!!! plz hlp:) how can i prevent this from happening? remember, i can only get one cage. Dont tell me to save up b/c i cant. dont ask why b/c its a long story. thnx!!!