Here's two great ways to get twins - they only work with Open For Business!

1.) When a Sim goes into labour and you hear them screaming, do Ctrl + Shift + C to bring up the cheat bar. Then type EXACTLY 'forceTwins' (minus quotes) and then when the Sim stops screaming a box will come up saying, 'There's TWO new babies in the family!'

2.) This is a bit harder, but if you 'don't believe' in cheats then all you simply have to do is get 2 Sims to do WooHoo, and wait for a lullaby after (this may take several attempts). When you finally do get a lullaby, get the pregnant Sim to CONSTANTLY eat cheesecake. When she goes into labour she should get twins.

Hope this helps somebody!