Block inside another block
From the url /admin/structure/block you can set Blocks to Regions on your site.
You simply drag and drop the block(s) to the given Region of your page and Save the block ordering on that page. In this screenshot I have 3 blocks in the main content area for my theme:



You can then go into each block with the configure link and set that block to show only on certain page(s) for instance, or only for certain Roles of the site.

This is the simplest means using Core drupal and no 3rd party modules.

If you want to eventually look into other means of doing this look into the Context module, or Panels.