How to make a blog with multiple tabs that support multiple posts

Create the main blog and set its title to Mainsomething:

mainblog.bblogspot.com

Create secondary blogs:

aaa-mainblog.blogspot.com (title: Sometitle1)
bbb-mainblog.blogspot.com (title: Sometitle2)

For each blog, go to Layout > Add a Gadget > select Pages > set the home page name:

For the main blog (home page) leave the Home name.

For the second tab/blog, change the homepage title to Sometitle1.

For the third tab/blog, change the homepage to Sometitle2 etc.

Edit the new Pages gadget and add external links for all other tabs/blogs.

On the main page blog, add external links for:

Title: Sometitle1
URL: aaa-mainblog.blogspot.com

Title: Sometitle2
URL: bbb-mainblog.blogspot.com

On the secondary blogs (e.g. Sometile1), add external links for:

Title: Home
URL: mainblog.blogspot.com

Title: Sometitle2
URL: bbb-mainblog.blogspot.com

…and then save the change.

Once all pages are added to all blogs, go back to pages and organise the order of tabs so that all tabs appear in the same orger in all blogs: Home : Sometitle1 : Sometitle2 etc.

Select the same template for all blogs > Customise > Apply to blog.