I believe the new thing since Generations is that Sims with the Family Oriented trait are more likely to have twins than Sims that do not have this trait. Of course, in a official Dynasty you can't change the traits of the founder or each generation's heir but you could give the spouse this trait just to give it a try.
I think the key is to try one of the 'may increase fertility' activities, not all of them: Family Oriented trait, Fertility Treatment LTR, listening to kids radio, watching kids TV.
In my Dynasty, I had my female listen to kids music or watch kids TV, and I had my pre-planned twins both times.