Having 2 WordPress blogs auto post on another (same domain)

Hello,

I have a question regarding the ability to have multiple wordpress blogs on the same domain. Is it possible to have the posts from 2 blogs show up on another blog. So having posts from site1.example.com and site2.example.com show up on the main domain example.com?

Please let me know if this is possible and how it can be done.

Thanks!