Jeff and I have also experienced this with our ferrets.  We got Hershey
as a kit and tried to introduce her to our existing pair, Ben and Jerri.
Ben was okay with her, Jerri was not.  So Hershey stayed in her own cage
and had her own playtime.  Then, about a year later, we got Casper as a
kit.  We introduced the two of them and it was love at first sight.  We
had other ferrets over the years, and Casper was the only other ferret
that Hershey would tolerate.  She slept cuddled up together - usually
hugging - in a hammock and played together wonderfully.  We could let
Casper out with the whole group, but not Hershey.  After Casper left us
for the Bridge, we gave Hershey some time and tried again with no luck.
She was in her own cage and had her own playtime until it was her time
to go find Casper at the Bridge.  I always envision them side by side
exploring the wonders at the Bridge and sleeping with their paws around
each other in a big hammock.
 
Kelley and Jeff
Loving Bailey, Shadow, Bear, Sammy, Justin and Daisy
Missing Ben, Jerri, Hersey, Casper, Felix and Oscar
[Posted in FML issue 4895]