Here’s how you can add both your Business and Personal Instagram accounts.
If you want to add an Instagram Business Account, please know that when you add your Facebook account, it should automatically add your attached Business Instagram accounts under the Instagram logo to the right of your Account Preferences.
If it doesn’t show up in the list, you can click the blue ‘Add account’ button under the Instagram logo and add it that way.
This is also how you add a Personal Instagram Account. Here’s a demo.