According to terms and conditions twin sharing does not mean that you dont have to pay. If you are using your credits then you will get more discount and other person have to pay more than you.
Like if Any trip cost is 50k then after credits used you need to pay like 20k only and your partner need to pay like 30k.
Twin sharing package allows you to share the package with the other person which doesn't means that you dont have to pay for your charges. Yes,you can enjoy the benefit of your credits which means that you dont have to pay the exact amount rather discounted price for the same trip.