Joyce,
 
First off, I am not a vet. Nor do I play one on TV :)
 
Vaccine effectiveness can be a sticky subject.  What I would suggest first
is to ask your vet of choice what he thinks, and or possibly call the
manufacturer of the vaccine (United Vaccine makes Fervac-D, for example,
and the # is 800-283-6465).
 
My personal choice would be to extend one ferrets vaccine period by 2
months, and shorten the others by 2 months, the next time you go the vet.
I don't believe that this will harm either ferret, or dilute the protection
provided by the vaccine, but ask the vet to be sure.  I would take a vets'
word over my own.  So, if one is due in June, the other Oct, take them in
August.
 
Good luck!
 
Lisa, Head Ferret
The Ferret Haven "By-the-Sea"
http://www.kozykingdom.com/ferrethaven
[Posted in FML issue 2440]