You first have to set up & login to your YouTube Publishing preset. The twitter preset simply takes the protocol from the YouTube preset & posts to Twitter. Hence the reason why their is the 'select a preset' dialog option in the Twitter configuration dialog.
To keep the selected clip from posting twice in a row on YouTube, you then simply uncheck the YouTube post option. The Twitter preset will then 1.) Post in YouTube & 2.) Embed the link in a twitter post.