I have two Joomla 3.0 sites installed on the same hosting account, each in their own directory on their own database. Both are subdomains of the same domain. The are hosted with Bluehost shared hosting at the moment, but I will be upgrading to VPS hosting once I go live. 

Hosting environemnt:

Apache version 2.2.25
PHP version 5.4.17
MySQL version 5.5.33-log
Architecture x86\_64
Operating system linux

Site #1:
subdomain1.domain.com
- is an online magazine with login/registration and ability to comment on articles. I am using the Zoo component for content articles rather than the Joomla default.I am also using the JFBConnect component and SCLogin module.

Site #2
subdomain2.domain.com
- is an installation of Jomsocial 3.0 with login/registration only, also using the FBConnect component and SCLogin module.

Both are using the same responsive Yootheme warp framework template. I intentionally installed them in different directories under the theory that if one were to malfunction, at least the other would be viable in the interim.

My question(s): 

Is it possible for users to login and register just once in order to be recognized by both sites/databases and have full use of all functions in both?

Is my installation of Jomsocial on a separate directory the best course of action, given my rationale?

Are there any other considerations to the above scenaio that I might have overlooked?

Thank you in advance for your help!