Q: Is it possible for two people to be logged into the back-end of WordPress at the same time?
A: Yes, it is possible for two users to log in WordPress if they use the same account. They can connect on the same WordPress sub-domain using the same credentials (user and password).
For example on the following sub-domain http://trailers.wp.tikilive.com/ (where trailers is the user who owns the WordPress blog) two or more people can log into the website, from different locations, at the same time, using trailers account credentials.
Please note that it is not possible for two users to log into the sub-domain (http://trailers.wp.tikilive.com/ ) if they use different account credentials. By default, WordPress only allows a single user account to be associated with a specific email address.