As spotted by multiple users including myself, Instagram has begun adding the ability to share posts to multiple accounts at once. Before, you could only share posts to one Instagram account and a few third-party social media profiles. Now, you can share an image directly to two or more Instagram accounts at once.
When sharing a single image, you’ll see other accounts connected to your smartphone as available outlets to share to. Just toggle which ones you want, hit share, and your post will go live across your Instagram accounts. Note that the function doesn’t work when sharing multiple images. It doesn’t exactly make sense, but that’s just the way it is.
Right now, multi-account sharing is only available for Instagram users on iOS. Those with Android will likely get the feature in the near future.