On most airlines, children can travel in their guardian’s or parent’s lap up to age 2. This means that no separate seat is purchased for that child. However, the caretaker has to meet an age requirement and can only take care of one child at a time. Seats would have to be secured for any additional infants. That second infant would then have to be secured in the seat.
Most airlines do offer the choice of purchasing a seat for your infant as long as you have the proper child restraint device. This could be either a car seat or a CARES harness.
When you already have your hands full with one in your lap, do you really want the hassle of dealing with a full car seat for your other child? CARES harnesses fit neatly in a small bag, are light, and take a lot of the stress out of taking a trip, especially when dealing with multiple kids.