How to setup shared content between main site and subdomain?

I'm setting up a WordPress site for internal use at my company. There will be a main site (mainsite.mydomain.com) and a secondary site (secondary.mydomain.com) that's a subset of the main one.

Similar questions have been asked, and the closest solution I've found is Shared Content but “Updatable” Through the Main Site. His solution uses WP multisite and a plugin called ThreeWP-Broadcast, which is unfortunate because the WordPress codex says...

For another example, if all you want is for different groups of users to have access to different information, then you can probably achieve what you want in a single site by using a plugin to switch capabilities, menus, and link URLs.

That is almost exactly what I need, but I've not found any plugins that do what the WP codex suggests should be done in my situation. So right now my options seem to be either...

1. Ignore WP Codex advice and use a multisite solution.
2. Write my own plugin/solution.

Here's a more detailed breakdown of what parts of the two sites are shared and which ones are site exclusive.

Shared

• A subset of the mainsite will be almost the entirety of secondary's content.
• Both sites will be on the same server.

Site Exclusive

• site URL subdomain
• menus
• site title
• theme (possibly just some color changes)

I have a little experience building and maintaining WordPress sites and PHP programming experience. I have full access to the server, which is Apache. Advice would be appreciated.