Force Twins
To force your Sim to have twins, there is a cheat known as "forcetwins." To enable this cheat, you will need to have your Sim already pregnant by the "Try for Baby" method. Once your Sim is pregnant, select your pregnant Sim; open the cheat box by pressing "Control," "Shift" and "C"; and type into the box "forcetwins" -- without the quotation marks. This will force your Sim to have twins when she gives birth.
Let Nature Take Its Course
"Sims" was created to mimic real life, to an extent. Like real life, twins do occur naturally. Every once in a while, your Sim will have a set of twins on her own, after you make them "Try for Baby," of course. However, this is very rare. This option is more likely to occur if one of the parent Sims is a twin.
Eat Cheesecake
If you have "The Sims 2 Double Deluxe" pack and the "Open for Business" expansion pack, you can make your Sim have twins by eating cheesecake. For this to work, you will need to have your Sims eat cheesecake and then "Try for Baby." If your "Try for Baby" is successful -- you'll hear the lullaby music afterward -- your Sim should be pregnant with multiples. If it is unsuccessful, you will need to eat cheesecake and "Try for Baby" again.
Hacks
There are many hacks -- special coding -- available on the internet that will allow you to modify your Sims game. Some of these hacks will allow you to have twins, triplets and even quadruplets.
A word of caution should be said about these hacks: While there are a lot of legitimate hacks, there are just as many hacks that will give your computer a virus when downloaded, so use caution when trying this method.