Pat,
My advice would be to supervise but unless it gets too serious (like severe
shaking, loud screaming, or blood) let them go.  If you are having any doubts
as to their play, seperate them, put the apparent victim a few feet away and
see what happens.  If the "victim" jumps back into it, it was play.  If the
victim runs away, it's time to seperate them for a while.
 
You might switch bedding between the two cages at night to get them use to
each other's smell.  If it gets really vicious, a little Bitter Apple rubbed
on the victim's neck and ears will often deter the attacker.  Most of the
"fighting" is probably just ferret play.
 
Dick B.
 
[Posted in FML issue 0946]