Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
If it is confirmed that there is a treatment that makes a woman give birth to twins without any side effects, then there is no harm in taking this treatment.
However, our advice is that a person should be happy with whatever Allah has decreed for her, and she should not interfere with such a delicate matter, because interfering in this matter could lead to some dangers like the deformation of the foetus, or that the latter and its mother be subjected to some serious physical harms.
So the above ruling applies to receiving a treatment. But if conceiving twins is done by way of artificial implantation, or what is called tube-babies, then this is not permissible except in case of necessity.
Please refer to Fatwa: 83776.
Allah knows best.