best way to copy posts from one WordPress installation to another?

Pretty much just what the title says. I’m working on helping a business update their main site to WordPress and consolidate all their blogs. Right now they have WordPress blogs at https://www.sitename.com/blog1 and https://www.sitename.com/blog2 and they would like to copy those posts so that all are available on https://www.sitename.com (as separate categories).

I found this plugin, but it says it’s compatible up to 2.8.4 and we are using 3.4.

https://www.ads-software.com/extend/plugins/copy-post/