What's the best nutritional plan for an expecting mother of twins in the second trimester?  all indications of a normal pregnancy at this time.

Healthy natural food. See my healthtap guide: feeding yourself in pregnancy. Healthy food usually means low in fat (especially low in animal fats), a moderate amount of protein, and a moderate amount of whole grain carbohydrates (but very little sugar). Natural means unprocessed, with little salt, sauces, or preservatives. Salt may increase water retention or swelling. Hot sauce and sour foods may worsen heartburn.
A variety. Eating a good variety of foods is most important, but unless there is a specific problem - gestational diabetes for example - there is no perfect plan. A daily prenatal vitamin with folate (folic acid) and iron is also advised. Of course avoiding alcohol and smoking is a must.
Rest up... My advice is to eat a healthy, well balanced diet and take your prenatal vitamins as prescribed by your ob/gyn. Indulge your cravings when you have them; but do so in small amounts if they are 'extreme'. Drink plenty of good old fashioned water; and try to walk as much as possible. Rest up; because caring for newborn twins is going to be taxing; enjoyable but taxing. Congratulations!
Approx 400 cal/d. A normal singleton pregnancy requires about 300 extra cal/d with twins requiring slightly more. The usda has a great website www.Choosemyplate.Gov which includes a dietary planning program www.Supertracker.Usda. In general for twins about 2400 cal/day total with a good mix of lean protein, carbohydrates, unrefined grain, fruits and vegetables. Enjoy!