Already answered in another newsgroup to which you posted the same question.

If you feel you need to post to more than one group (HINT: it's seldom
necessary), please have the courtesy to cross-post (send the one message to
all groups at once), rather than multi-post (send individual messages to
each group). In this way, all responses to your post will be available
together, regardless of what group the responder was in, and the rest of us
won't have to read your post multiple times. (It also uses fewer server
resources)

I'm afraid I'm not familiar with the Opera news reader, but most other
newsreaders have a way to let you add additional newsgroups to a post.