No. Carbons are only doing this:
User 1 sends a message to user 2 in Conversations. This same message is showing as sent in Gajim also (in its history, in the chat window).
It also works the other way round:
User 2 sends a message from Jitsi to User 1 (bare Jid). If both resources (Gajim and Conversations) have XEP-280 enabled they both receive the message.
If user 2 instead sends to a full Jid, let's say to Conversations, Gajim would receive an "<forwarded/>" copy of it.
Given that both resources have XEP-280 enabled.