My sister and I had birthdays within two weeks of each other and it was never really an issue because my parents never made it one. Their rule was simply this. on the even years (like 1990 or 1988) we could choose to do a really special activity and go somewhere but we had to choose something together.
Choosing together allowed us to do really special things like the water park or a camping trip to a cool little island with a handful of friends. If we had a regular old backyard party we could choose to have them separate, but anything that meant going somewhere meant having it together (less expense when there was only one of them to do) My sister and I took full advantage of this.
As we got older, mom and dad even gave us the budget and allowed us to plan our own parties and whether it was an odd or even year, if we could figure out how to do something together, work together and not have arguments that needed to involve them, and get it done we had full control. It was great. The fact that we could also choose separate birthdays helped too because then we had the choice (which meant we never resented having shared parties).
The only time it became a problem was when I was old enough to want a little more grown-up party during adolescence and she was still young enough to want cartoon character things and themes (I, like your sons, was three years older). That particular year especially, but others as well, may have led to the kind of resentments William Grobman mentions in his answer. Instead, it became one of the most memorable because Mom and Dad introduced the idea of the camping trip which gave me, my "sleep-over" and her the pirate theme she wanted because we hunted for buried treasure as part of the party activities and we were on an island. It was still our choice to do that together or have separate, simpler parties and it worked out well. I think if the choice had just been made for us, mom and dad may not have been so creative and I may have wound up waiting for the sleep-over part a year or two, too long harboring resentment.
The key thing I would remember as they get older, is that convenience for you, may mean sacrifices they are unhappy with when they are older. Your children are two different kids that will likely grow to want two different things. One may like huge groups of people and want big noisy celebrations while the other will prefer movie night with a couple of closer friends. . . Whatever you decide to do, you will make things easier for yourself if you find a way to include them in the planning and decision making. If they are part of the planning then compromises are ones they made to make something they did want work and they won't resent them. I do think there are times when negotiation is not a good idea between parents and kids, but when it comes to their own birthday party, I think a little negotiation is absolutely crucial.