If you want to have baby girl, many people say don't have sex on the day of ovulation, but have sex 1-3 days before ovulaton.
If you want to have twins, unless your family or your c6's family has twins history, otherwise it is very hard. Or you can try IVF, though IVF treatment, you can transfer 2 or 3 embryos inside you tummy, then better chance to have twins or even triplet. But my doctor said it is better to have 1 baby at one time, twins are easlier to cause healthy problem to both mother and babies.