Instagram has a built-in feature that enables you to be logged into multiple accounts and switch between them easily (similar to Twitter), so yes it’s ‘legal’ and not against their terms of service. To use the feature, go to your profile in the app and click on your name at the top left.
What happens after 10k followers on Instagram?
Once you have 10k followers, Instagram will make it easier for you to get people to your website via Stories with the swipe up to link feature. Swipe up is the only way to get a direct link from your Instagram to your other web properties. And, it’s available in STORIES, if you have 10k followers.