You should start Taking folic acid 6months before conception, if you really want to conceive twins.. Though sometimes it comes naturally. I used mine for 6 months before conception. My husband too was taking some medications to help as well, cos you also need an active sperm.
@Hannah, do you have twins after taking the folic acid for 6months? I have been taking folic acid since last year but not daily. Will it still work for me since it's not taken daily?