Apps like Hootsuite or Buffer allow people to post one piece of content to multiple social media platforms. It's a user-level automation tool really. Say, you write something and connect your Twitter and Facebook account to HS or Buffer, and post to both. Twitter and FB don't directly share this and have no cross-posting between them - for them you're basically posting just like any other user. 

As a community operator it's a bit different - if you're trying to post one item to multiple sites it may be tricky - think of situations like users not having correct permissions on different sites, or not being able to choose contexts, or contexts don't match, etc. I assume your sites are not exact clones of each other, so there may be quite a few considerations. Perhaps we need to look more closely at the task at hand.