You can publish content simultaneously in multiple channels using and "Inject" or "also publish". They're really the same thing except Inject is specifically for the Facilitator channel.
Adding an "also publish" block to your content
When you select a content item you'll see a + appear beneath it. Click this to add a "also publish" block.
When ANY button is clicked (or trigger fires) on that content item, any content connected to the "also publish" will fire too.
In the example below, clicking "Get documentation" will result in publication of content 18 in the same channel (Msngr) and the simultaneous publication of a PDF in the Library channel (content 16). This is because content 16 has it's channel set to Library and it's connected via the "also publish" to content 15