See your eyes😀. Unfortunately there is no scientifically based natural method around this dear. Gynecologically...at the moment, the process of having twins is invasive and financially intensive as it will involve invitro manipulation of interaction between sperms and eggs.
But naturally, some studies have found that yorubas are world's no 1 in terms of birthing twins. Again ladies that are well advance in age are more likely to give birth to twins.