If I were the OP asking that question I would anticipate that most people who reply would have eaten at one or the other and there may be a few who have eaten at both. I think you still get a reasonable comparison even if some people review one restaurant and others review both. If the idea is to choose between two restaurants I would think that a larger volume of reviews of both restaurants would be most helpful and if only people who have eaten at both respond, you are limiting responses. I guess what I'm saying is if I asked for reviews I'd be interested in all reviews.