As MinnieMiss posts, in most cases you can. I would encourage you to call the airline PRIOR to hitting the "Purchase" button so that you can advise them what you are doing and they can note the record locator before the reservation is flagged for security reasons.
I did it for a trip to Arizona in 2009. I flew American Airlines and called to see if I could do it. They said I would have to call them to make a 2 seat reservations for one person. I did and there was a small fee for doing it that way. Southwest has a policy that you can buy 2 seats online. I don't know the exact way to do it but it's laid out on their website. You just go to the Passengers of Size link on their site. It's fairly easy from what I read. Southwest also has a policy that you can reimbursed for the second seat after you fly with them. Hope I helped.
You can book an extra seat on JetBlue. In the section where you fill out the passenger info you can select "This is an extra seat for" and put your name in the field.
Yes you can, but I'd recommend calling if the airline doesn't have a specific "extra seat" option on the website, as your duplicate seat could get purged from the system if it's not correctly blocked.
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