Try to arrange to travel at non-peak times, avoiding holidays, weekends, and other busy travel times. Consider your twins' schedules: When are they most rested and content? When are they crankiest? Some families find that flying late at night or overnight works best, because the twins are more likely to sleep during the flight.
When you're booking your flight, pay attention to the flight's itineraries and details. Direct flights may stop along the way, even if you don't have to change planes. Nonstop flights may be a simpler option. Don't rule out commuter flights. Although the planes are less roomy, there are fewer passengers to contend with.
As much as possible, book EARLY! You'll have more flexibility with flight times and better seat selection.