If you use the same mobile number to turn ON 2FA on a second instagram account it DISABLES 2FA on the first instagram account.
So you need different mobile numbers for each instagram account you have to avoid getting the following warning :
You turned off two-factor authentication for this Instagram account because you reused the same phone number to confirm another account.
This means you won’t need to enter a security code when you log in.
If you want to turn it back on, you’ll need to use a unique phone number for each account.