What is the best way to align the subwoofers for summing?
The summing of the outputs of 2 cabinets offers the same benefits directly on-axis regardless of their orientation. It's off-axis that there are some differences. In a VS21, the lowest frequencies come from the ports and the upper frequencies come from the mouth, with the curved grille. Destructive interference starts to occur when the sources are separated by more than 1/3 of the wavelength being produced. For this reason, the best coverage is achieved when the horn sections are together. The lower frequencies coming from the ports are still within the minimum distance at just under 6' apart. That said, putting them the opposite way, with horns out, won't cause any problems and, although the coverage above 60Hz might be a little less even, you might like the sound that way better, so don't worry, you can't get it wrong.