If you want your sims to have twins get the pregnant sim to eat cheesecake during pregnancy or before you woohoo (You need to have Sims 2 Open For Business). I've tried this and it's worked every time. Another way I haven't tried yet is type in "intProp chanceoftwins 100" in the cheat bar (Which you open by holding down shift, ctrl and c). I haven't tried this yet, but I have tried "intProp chanceofboy 100" and it's worked twice. Hope this helps.