Combining content for a "master" site?

I’m currently running 2 individual blogs that will soon become 4. They are all part of a series of blogs on similar topics – the issue is we want one “master” site that has all the content from the other 2-4 blogs.

So we have:
site1.domain.com – unique site
site2.domain.com – unique site

And we want:
https://www.domain.com – all content from the above 2 (and in the future 4) sites.

I’m looking at doing this with custom code but that’s going to be quite a pain as the data is in two separate databases (and I need/want the data merged so sorting by timestamps will work correctly). I’ve read a bit about Multisite but from what I read it’s not necessarily what I want to use.

I’ve considered writing something that would just suck in the RSS feeds from the other two sites and display it but that wouldn’t bring over things like comments, etc.

Suggestions?

Thanks!

-Ross