Micro.blog for Mac supports using multiple accounts. You can add new accounts in the preferences window and switch between them by clicking on your profile information.
On Micro.blog, each account has its own username. Accounts can also have multiple blogs. This is convenient if you want 2 or more custom domains with different titles and content, with posts from each microblog coming into the same account.
(If you need multiple usernames, you should create a new account instead of a new microblog under an existing account.)
To add a new hosted microblog, click “Plans” → “New Microblog”. You can change your default microblog by clicking “Switch Site” on the web posting form. Micro.blog for Mac can post to a different microblog by clicking on the hostname at the bottom of the posting view.