Edit: Far as I know, Pam is the Queen of Nooboos, so she might have some suggestions for mixed gender children.
Thank you, Shirin.
The main thing is that EA changed the code in the game not long ago and made it to where mixed gender twins and triplets are quite common. You don't have to do anything special to get both genders. The trick with the doctor will still work, but it was developed back when multiple births were always the same gender and we were forcing the mixed gender to happen. No need for that now. As for getting multiples, it can happen even if you don't do anything at all, but it's not common. To help the chances for twins or triplets, use the fertility treatment Lifetime Happiness Reward, watch kid's television or listen to kid's radio. Personally, I've never had the radio do anything, but it's been confirmed by sources I trust. The more of those things you do, the more chance you'll have of getting more than one baby. Also, the more of them you do, the more chance you have of getting triplets. For example, if both parents have the fertility treatment and the mother spends a couple of days in front of the television watching the kid's channel, she will likely have triplets.