Make sure there is adequate iron and that you are gaining an adequate amount of weight in the first trimester, which can be difficult with morning sickness. There is a higher rate of premature delivery early in the pregnancy. I would make sure the prenatal vitamin you are taking has zinc in it. Make sure you are drinking three to four glasses of milk if you can, but if that is not possible, then supplementing with calcium would be important. By 24 weeks with one baby, mom is gaining about 12 pounds, whereas with twins, you want her to be gaining between 24 to 30 pounds.
Twins can have either one placenta, which is most common, although they can have two.
With fraternal twins, they most commonly have two placentas. There are some situations, that are rare, when the layers of the two fetuses grow together and there is one placenta. The placenta nourishes the fetuses. There is no unsafe time, in terms of air pressure, because that is pretty standardized now. In general, most doctors won't want their patients to be traveling after 32 weeks.