Generally writing the same thing on more than one of your sites is not such a good idea.
But especially if you have custom post types, for example for products that you are selling, they may make perfect sense on more than one of your sites.
With WordPress Multi-Site, making more than one site (in the same installation of WordPress) is very easy.
How do you put a post on more than one site? Of course you can copy and paste the main text, and then manually enter all the other settings for the post (for example, SEO settings, and the many settings for your shopping cart system), but is there a better way?
There is a Multi-Post plugin, http://wordpress.org/extend/plugins/multipost-mu/ but warning this plugin is no longer updated, probably should look for another.
“Allow a WordPress MU site administrator to duplicate posts and pages to multiple sub-blogs at once.”
Gives a simple list of checkboxes on the Post/Page edit page, for which other sites to also post to.
Modify the Original page, and the copies reflect the change. Modify the copied one, and it is generated as a separate post, no longer linked.