Use the Fertile reward trait from the Satisfaction Rewards Store
Have one or both parents drink a Fertility potion
Make the mother watch the Kids Channel on TV before trying for a baby
Make the mother listen to the Pop station before trying for a baby
Use the On Ley Line lot trait
Try for a baby while the mother has the Fertile trait and the On Ley Line lot trait active
Use mods or cheats that increase twin pregnancy chances
Save before trying for a baby and reload if twins do not occur