You seem pre-occupied with frequency response over bass smearing issues from using two subs. The relatively simple task of moving one sub and treating the room so the listening position is both time coherent and within tolerable frequency responses is a far more reasonable a solution. Most speakers don't need a dedicated sub, especially if you are dealing with small rooms. When you say better, you mean flatter, but that's not the only thing to consider and I would argue it's has less value than you are currently placing on it.