It's actually pretty easy to delete a second Instagram account- and here's how:
First, go to your profile page on the app. Second, click on the three dots in the upper right corner of your screen. This will bring up a menu of options. Next, select Settings from this menu. Once you're in Settings, scroll down until you see Account. Tap on Account, and then select Delete Your Account. You'll be asked to enter your password to confirm that you want to delete your account. After you enter your password, select Permanently Delete My Account. And that's it- your second account will be permanently deleted!