In the database, every blog stores its posts/pages separately so sharing content become quite tricky.
A few things to look into:
Sitewide Tags plugin. This copies any post (and now can do pages as well) to a designated "tags" blog. So that blog can act as a portal to the content in all the other blogs. The permalinks are retained as metadata to link to whichever blog originally wrote the content.
Page Links To - I've used this to create placeholder pages/posts on Blog X that links to the actual content on Blog Y. So for instance, one blog might be articles while another is a product catalog. I create tabs on both for "Articles" and "Products" and using that plugin can link those tabs to the appropriate blog.
Embed RSS - Lets you create a page on one site that shows the RSS feed from another site. This can be cleverly combined with category level feeds.