If your accounts are linked, you will be able to switch between them or see them in a consolidated view, using the account selector present within the app.
If you have two or more independent (not-linked) account, you have as well multiple usernames (at least one for each account). Our mobile app does not support simultaneous user sessions, hence you would need to terminate a session by logging out the user before starting the next one.
As a workaround, you may user our Desktop platform, the Trader Workstation (TWS), which allows you to launch multiple instances and log in with different usernames. Mac users may refer to KB2637 for information on multiple settings.