On the incoming side Yarn identifies articles through the message-id.
Thus you will never see artciles twice.
On the outgoing side it is no problem to post an article to each NNTP
server you are allowed to post to. The article will be distributed
worldwide - as long as the groups are public.
Special meachanisms are not available for Yarn. I suppose your
problem will be, that there is only one output file (i.e. news.msg).
You could write a small program which filters the articles in the
news.msg according to the newsgroup header in two seperate files and
deliver those files to the corresponding servers. You could also use
Souper/VSoup for your purpose, if you specify a special POSTER program
(you have to write it yourself), which would seperate the messages for
your two servers into dedicate files.
-- Hardy Griech, Kurt-Schumacher-Str. 25/1, D-72762 Reutlingen