You can even use the same .jnewsrc for multiple NNTP servers, so just running `NNTPSERVER=some.url slrn` works for me. But I'm hoping for a way to use multiple servers without re-launching the client.
EDIT: the subscribed groups need to exist on all the servers, though. Otherwise slrn drops missing ones.
alias slrnreddit='NNTPSERVER="news://localhost:8119" slrn -f ~/.jnewsrc-reddit -i ~/.slrnrc-reddit'