Duo and Trio blocks on Landing Pages are a way to specify 2 or 3 pieces of content to be shown together.
First, the specification of the content is done while editing the landing page itself. You can have more than one duo or trio per landing page, but I only have support for up to two of each so far to be displayed.
Then, within the layout of the landing page, you add one or more of the view with a name like Trio: First Block, Trio: Second Block, etc. These views are 'context' aware blocks that generate the block content based on the configuration of the page you did above.
Generally, you won't be creating new trios/duos, but just updating the landing page to show a different piece of content in one of the trios/duos.