Hacky method to show content from multiple Grav instances

I’m making a multisite Grav where each subdomain has their own Grav instance. I’ve been cracking my head over how to have the main page show a collection of posts from the other subdomains since they are on their own and I found a neat trick.

Since Grav is flat-filed, I created a shortcut file in the pages folder my main site linking to the pages folder of the subdomain.

My main Grav instance reads the pages like it’s own and generates them in the main posts page.

This is of course a problem with SEO. Does anybody know of the official/proper method of doing this?

Have you tried symlinking certain pages from one site to another?