How to merge two individual blogs into one?

I believe the only way is to create a new blog on your 3.0 install and then Tools -> Export and Tools -> Import the blog over.

Thank you
In this way, How to transfer the attachment files?

When doing an import, there’s an option to import the attachments as well. However, this works directly between the two sites, meaning that the old site must be still alive and visible.

Also, importing doesn’t change the links to images and things to point to the new attachments imported, so you may need to use a search and replace plugin to fix those links after the fact.