Okay, first rule of thumb: whenever a pet gets a vaccine ALWAYS wait in
the veterinary office for at least 45 minutes.  Waiting much longer is
better since pets can not tell you is an eye is starting to get itchy, if
a hidden hive may be present, if swallowing is becoming difficult, or if
other early signs of a reaction are happening.  (This is the same reason
allergists have people wait 20 minutes in their offices after allergy
shots.)
 
Not being able to get immediate veterinary care can reduce the chances
of good outcome, but fortunately, ferrets do usually survive these
anaphylactic allergic reactions.
 
Since both shots were given in the same visits you are stuck in the
situation where you don't know which vaccine the ferrets reacted to.
It's usually the CDV one, but not always.  For that reason it is almost
always advised that the shots be given at separate times, and to reduce
the challenge the ferret is facing it is almost always advised that
those visits be separated by at least two weeks.
 
It is also usually recommended that ferrets be pre-treated with Benedryl.
Some vets prefer their clients to do this ahead of time, while other vets
prefer to give a Benedryl shot themselves beforehand.
 
The new Merial Purevax ferret canine distemper vaccine has in field uses
as well as the pre-release studies shown itself to have a greatly reduced
risk of causing life-threatening allergic reactions.  Yes, some still do
react; that is true or anything.  Some people die if they eat common
foods, or get commonly taken medications due to such reactions.  The
reduction in risk is far better for ferrets, though.  It also is
important that folks do their best to make this economically a worthwhile
venture by Merial to encourage further medical improvements for ferrets.
[Posted in FML issue 3873]