I'm curious to see what others have to say about this question.  It's a
catch 22 .  I'd be terribly afraid to introduce a new ferret into the mix
with the one being such a Senior Citizen.  If the new ferret happened to
be an ECE carrier ( you won't be able to tell looking at it and can't
test like for ADV), it could absolutely do the older one in.
 
On the other hand, I know what it's like to have the last survivor, and
how depressed and lonely they can become.
 
In my mind, it's not worth the risk to the old boy and although the
survivor mig ht go through an mourning and adjustment period, I don't
think that's quite as m uch of a problem as the potential risk to the old
one.  I personally don't like to expose ferrets who have never had ECE to
any new ferrets after age 3.  The younger ones seem to handle a bout of
ECE better than the older ones, and even with that, we're seeing so many
things that appear to be longer term effects of ECE than just getting
them past the green slime phase.
 
J Carel, The Ferret Haven, Louisville, KY
[Posted in FML issue 4058]