From b2evolution manual

One of the strengths of b2evolution is that it is able to handle multiple blogs out of the box. As a matter of fact, on most installations it will come with multiple blogs pre-installed.

[edit] Multiblog Scenarios

Multiple blogs can be used in several different scenarios:

• One website with multiple blogs accessible through different names - blog1.php, blog2.php, blog3.php, etc. This is the default setup, you can then change the names of the php files to suit your needs. Each blog can also have a different subject, purpose or even content type - i.e., podcasts, or photoblogs.
• One website with multiple blogs scattered through a complex tree of subfolders.
• One domain with multiple blogs accessible through subdomains - blog1.domain.com, blog2.domain.com, etc. Again, you can choose any name to suit your needs.
• One b2evolution installation serving multiple domains.
• Any combination of the above.

[edit] Using Multiple Blogs

• Sample blogs : Blog A & B, Linkblog...
• Using stub files to anchor particular blogs in particular locations of your website.
• Using summary.php
• Using multiblogs.php

[edit] Advanced Setups Involving Multiple Blogs

• Multiple Blogs on Multiple Directories
• Multiple Blogs on Multiple Domains