> The experienced individuals from the first list wishing to participate in
> the second list are among the most active in the discussion that take
> place in the first list.  They will want to be able to participate at both
> levels and will therefore continue their subscription and active
> participation in the first list. However, this would cause them to receive
> duplicates of messages originating in the first list and, more difficult
> for them to deal with, replies to the messages originating in the first
> list which they received from the second would not make it back to the
> first list.
 
Subscribe them to both lists, but set them to NOMAIL on the first list.
This will allow them to post on either list (I'm assuming that at least
the first list is set to Send=Private), but they will receive only the
copy from the second list.
 
Replies are trickier.  If you sent both lists to Reply-to=List,Respect,
then the posts to the first list will have a Reply-to pointing to the
first list when they appear on the second list.  This will be right
for the people on both list, but not for the people only on the
second list.  If you set the second list to Reply-to=List,Ignore,
then the messages on the second list will point to that list, and
people on both lists will not know which list the message went to
originally.
 
It might help to set the people on both lists to IETFHDR.  This should
preserve the To header when the message goes from the first list to
the second (leaving it pointing at the first list).