can two people use one line at the same time?

Hi Jerrald, 2 people cannot use the same line at the same time. If you disclose your OTP to someone, the person can link your line to his line via MyMTN app and can use it to access your line. You should not disclose your OTP to a third party to avoid this; however, if someone has linked your number, it will show on your on end and you can unlink yourself via MyMTN app.
When you login to the app, click on the dropdown menu, select Add number, you will see the button to unlink yourself.