What is a recommended way to manage different roles in a webapp?

Question

My app can handle Medics and Patients. So far everyone can login and perform different tasks on the site.

We want to avoid forcing people that are Medics AND ALSO Patients to create a new User Login and password.

So a Medic which is also a Patient should keep his same login and password but we are thinking that as soon as the person with this dual privilege logs in they should PICK which role (and maybe store in a session variable) he wants to log in as.

Is this recommended or can you please provide me with an alternative?

Please help. thanks in advance