That heavily depends on how the web site handles sessions. If this is done through a unique ID in every request as a request parameter, then this is possible. But as soon as cookies are used, this is not possible.
CasperJS is like a browser. You can have multiple tabs (casper
instances), but the session is handled by the browser (PhantomJS). Even if you have multiple casper
instances, they operate on the same cookies, so you cannot be logged in as a user and as an admin at the same time. This is because PhantomJS handles the cookies and even if you have multiple casper
instances, there is only one phantomjs process underneath.
If the admin interface is on another domain, this multiple casper
instances should work. The rule of thumb is, if you can login in a single browser window with multiple tabs, you can do it with multiple casper
instances.
So, if they are on the same domain and cookies are used, you would need to do this with different scripts.
Other questions that might interest you: