To view several RSS pages at once:
Click the open-book icon to show all your bookmarks.
Click the Add (+) button at the bottom of the left column to create a new “untitled folder.” Rename the folder with a name of your choosing.
Drag bookmarks for two or more RSS pages into the new folder.
In the left column, click either Bookmarks Bar or Bookmarks Menu.
Drag the folder of RSS pages to the right column.
Now you can click the folder's name in the bookmarks bar or Bookmarks menu, and choose View All RSS Articles.