Yes an admin and agent can exist in the same account with one email address and one set of credentials to login.
If you as an admin add an agent with the same email address as the one you use as an admin, your user will get the 2nd role as agent.
Note that this means you can login to babelConnect as an agent or to the management / admin interface as an admin. If you want to do both at the same time, you will need to have two sessions open, e.g. by opening an incognito window in chrome for one of the sessions.