I hope that this doesnt sound too harsh, but why are these kittys not spayed and/or neutered?? you would not have to worry about unwanted /un-necessary pregnancies if proper precautions were being taken in the first place. Please don't take this the wrong way, but I am sure many of us here would also agree that spaying and neutering should be of the utmost importance.
I have a male (neutered) and a female (spayed) who live very happily together and I have NO worries about any unwanted pregnancies.