How does someone get pregnant with twins?

2 ways. 1. A single egg and a single sperm create a single embryo that splits in two very early in pregnancy. 2. Two eggs are ovulated and each gets fertilized with a separate sperm.
They have sex. Twinning can occur from a few causes. Taking fertility drugs of having certain fertility treatments like ivf can increase your risk for multiple births. Other spontaneous causes occur when you release more than one egg during a cycle, or if after fertilization the fertilized egg splits in two - in this instance you would have identical rwins.