You can choose to transfer two embryos instead of one to increase the likelihood of pregnancy and thus increase the likelihood of a twin pregnancy. However, there is no way to ensure that both will implant.
Degree in Biochemistry and Molecular Biology from the University of the Basque Country (UPV/EHU). Master's Degree in Human Assisted Reproduction from the Complutense University of Madrid (UCM). Certificate of University Expert in Genetic Diagnosis Techniques from the University of Valencia (UV).