How to Enable Multiple Main Domain Blogs

Main Install: maindomain.tld
Blog: maindomain.tld
Subsites: subsite.maindomain.tld

I also have domain mapping to some of those subsites with domain mapping.
This part works fine already.

I want (on the same install)...
Blog: maindomain2.tld
Subsites: subsite.maindomain2.tld

I setup multi-domain plugin and created subsite.maindomain2.tld, but there is no blog for maindomain2.tld

My question... How do I setup the blog for maindomain2.tld ? Adding it as an addon domain seems like playing with fire.

Any suggestions?