On a lark, I decided to see if those behind the Boston marathon bombings had left a clue about the bombings' actual purpose (which I suspect was partly intended to symbolize the "war on terror" up to that point, as a means of announcing a phase-change) in the form of the distance between the explosions.
So, I determined the distance between the centers of the Twin Towers, and obtained about 450 feet, and then obtained a rough figure for the distance between the Boston bombings (one on left end of Marathon Sports at 671 Boylston, and the other in the middle of Forum bar/restaurant at 755 Boylston), and obtained 390 feet, although I wasn't able to view one of the sites (Forum bar/restaurant) on Google Maps satellite imaging, so I went by the address, which could have introduced an error of about negative 25 feet. (In other words, the distance between the actual bomb sites might have been about 415 feet.)
So, my rough measurements indicate that the Boston bombings were about as far apart as the centers of the Twin Towers. It doesn't prove anything, but it is tantalizing. Perhaps there would have been a better match if the distance between the sites were the only criterion.
I find it interesting that the second bomb exploded across from the Prudential Center, because Prudential was behind the choice of sites for British imperial aerial terror-bombings during WWII. Based on those studies, they were able to adjust their bombing patterns to minimize the amount of bombing required to drive the population out of their minds (actually vice versa). For more on these surveys, see Excerpts from, and comments on, The Shaping of the Anglo-American SS by War.