There are two options you either add another whole page of content to a Display/Agile URL or use the Multicontent element on a page and add additional content blocks within that.
Adding another complete page to a Display/Agile
If it is a complete page you are referring to go to the Display/Agile menu and click Manage Content on the display/agile link you want to add another page to.
Choose a page you want to add from the list on the left (this is a list of all pages that have been created, including any imported templates), it will then appear in the box on the right, click the down arrow to expand it and you can set the display time. You can have as many pages as you want in the Display content box, if you have one in there already just add another using the same method. Click Save and then refresh to push the pages to the display/agile.
Adding a new block to a Multicontent element
The Multicontent element allows you to create movement within a section of a page design to make more of your screen real-estate.
If you want to add an additional content block to Multicontent element go to Browser menu, click on the page you want to edit, select the Multicontent element, click the cog icon to configure. In the Multicontent element properties box you can either click the down arrow on one of the blocks to expand it, then click duplicate to copy it and set your display time and transitions, or click the add block button to create a new block from scratch within the Multicontent.
For more on Multicontent check out this article: Using The Multicontent Element