it was a tip sent to me in an official email from EA Games in reply to registering my game. Way back when, when the Sims 2 was fully up and running, if you registered your game, you got an official email from EA which included a tip, often a secret tip that's not told in the manual. Cheesecake = twins was one of them. So I tried it out for when there was only space for one baby in the house, and sure enough .. twins, tipping the household number to over the maximum. What happens to their portrait is it appears in a second column, smaller than the others. I've been told this is what the layout for portraits is for Pets. So if you have Pets and you've seen those smaller portraits in a second column, that is where the extra baby's face will appear, and they are fully operational.
Try it yourself, prove me wrong, but you won't. Your sim has to eat the cheesecake while pregnant. One slice is all it takes.