I've flown a lot in the last 5 years. At my top weight I was 300 pounds (and very big because I'm short) and few both domestically and internationally. I carry the a lot of my weight in my hips and stomach. I've only used an extender once and that is because it was left on the seat. (I was always to embarassed to ask for one so instead delt with being very uncomfortable.) I was never asked my weight or told to purchase another ticket.
The web site
seat guru provides a list of which seats are the best and how big the seat are on individual airlines. You might want to see what airline/plane has larger seats and see if you can book a ticket on that airline. It might be more expensive for the individual ticket but probable cheaper then buying two tickets. You also could set up a "seat" by measuring out the width and setting up something like books at the edge to see if you would be able to physically fit in it, if it is close then I probable wouldn't buy two because you can squeeze. But that is just my opinion.
Oh one more point if you are concerned that they will not have seat belt extenders they will that is what they use to do the demo of how to buckel a seat belt. I have heard that if you ask in advance sometimes they will already have them on the seat for you so you don't have to go throw the embarassment of asking.