thanks for taking a moment. Do I need user synchronization

Hi, thanks for taking a moment.
Do I need user synchronization to sync users on one site between the main site and their own blog or is this for completely unique urls?
maindomain.com & mydomain.com/user-blog
vs
maindomain.com & yourdomain.com
Thanks so much,
Y
I will have "mydomain.com" & "mydomain.com/user-blog" and would like users to have one set of credentials.