Aunt Cob: I definitely think you need to evaluate your doctors recomendation of only using airway clearance when your son is "sick". I'm sure that if you did it on a constant basis, your son would most likely raise his pfts... in the 80's isn't horrible...but it's not good either.
Idenkers: Just because your twins don't have any symptoms yet, doesn't mean they shouldn't be treated from the start. It's important to keep the secretions moving, so bacteria don't harbor in their lungs, and also to get them used to it. Cf is progressive, and they wil NEED the treatments to keep them well in the future. By doing them now, you get them used to it, and perfect your technique. Depending on their size, you might be able to get vests for them. My son started the vest at 1 yr. Since your sons haven't shown any symptoms, it would probably be hard to get an insurance company to approve one sooner. Even if you dont use a vest, manual cpt should be done daily. I'm not a doctor, but our clinic prescribed 2xs for 20-30 min when my son was an infant. now that he has the vest, he does it for 30 min, twice when WELL... 4 times a day when sick...and every 4 hours around the clock when in the hosptial.
If they have not taught you, it's time to ask the respitory therapist to show you how. There are videos online, or books you could look at.
If you need any more advice, or would like to talk, pm me. I have 3 with the same mutations as yours, and would be happy to help.