Use an alias for that:
alias slrnreddit='NNTPSERVER="news://localhost:8119" slrn -f ~/.jnewsrc-reddit -i ~/.slrnrc-reddit'
alias slrnreddit='NNTPSERVER="news://localhost:8119" slrn -f ~/.jnewsrc-reddit -i ~/.slrnrc-reddit'
EDIT: the subscribed groups need to exist on all the servers, though. Otherwise slrn drops missing ones.