Secret nikaah

I am currently engaged to a certain girl. We are completely in love with each other and go out together all the time. To save ourselves from Haraam we want to make nikah asap. However our parents are very adamant that we are still young and the nikah should be made 2 years from now, and they dont mind the Zina that is taking place. My plan is to make a secret nikah now and 2 years later I will have a normal nikah in the musjid. However, I heard a Mufti saying that this way of secret nikah and then open nikah later on is not permissible because it is haraam to make a 2nd nikah with the same lady again, it is playing around with Allah's Deen, is this true?