Or, it just so happens that they both ended up being at some stage of "ready" at around the same time, and add in that a release around mid- to late-October is good for sales leading up to the holiday season, and interest in computer games increases towards the beginning of the winter season because our northern friends are more likely to stay inside for entertainment than to brave the snow
And while I think that there is some overlap between potential players for SEV and potential players for DomIII, I don't see one cutting in on the sales of another. If anything, there would be a mutual increase, as the crossover players evangelize to the other player base.