Some iOS users are able to toggle between multiple Instagram accounts
According to some Instagram users running iOS, a subtle upgrade to the app is now allowing a small number of users to quickly switch between multiple Instagram accounts without having to log out and log back in. Last November, some members of the app's Android beta testing group were able to quickly switch between multiple Instagram accounts after manually installing an APK. The APK is not necessary for those with iOS.
Overall, it sounds like Instagram is testing out this feature before it becomes permanent. To see if you have the ability to open multiple accounts with your Instagram app, head over to your profile and click on settings. If you have the option to add an account, congratulations. Those with multiple Instagram accounts are discovering that when they receive a notification, it does mention which particular account it is related to.
source: Engadget via Latergramme
Many social-media apps already offer the ability for users to open more than one account, and quickly switch between them. Unfortunately, Instagram is not among them, at least not for everyone right at this moment. Based on these tests however, it won't be long before Instagram users will be able to post pictures of their meals on multiple accounts with a quick click of a toggle switch.
source: Engadget via Latergramme
Things that are NOT allowed: