How can we have a multisite that uses the same content?

We have a main website and want to make a sub-site of that but use much of the same content from the main website but a different theme and possibly some different plugins. But the main thing is we need to use the SAME posts, pages, and custom posts that are on the main website. The client should only have to update the main website’s content to change it in any of these subsites as well.

I figured we’d be able to somehow hack together a solution with Multisite but I can’t find one.

Any ideas? Anyone tried this before?