Multiple plane tickets somehow seem to cost more when you look for them at once compared to just searching for one ticket first. By doing that, it will ensure you the lowest available price for a seat in the airline's computer system.
I figured this out when I was booking one trip out, so I decided to give it a try and search for one seat instead of two. Airline seats are grouped into price buckets, and if you want to buy more than one ticket, there's sometimes only one ticket left in the lowest price bucket. So most airlines will quote you the price of both tickets in the higher bucket rather than the available lower bucket seat.
To save money, Jason Steele from MoneyTalkNews writes:
always specify one traveler until you find the lowest price. Then, search again with the actual number of travelers and see if the price changes. If it does, you can always book one seat at the lower price and make a separate reservation for the others at the higher price.
Note that this trick also works for booking award seats at the lowest mileage levels.
Check out the full article for more ways to save money when booking your flight: