Some people make two websites for same niche or product because they want to test their sales copy. And which ever sales copy get them more sales, they use that copy. But very people do this. Most people make multiple webpages on same website to test their sales copy. But people do have multiple websites for different niches and products. Having multiple products of different niches only confuses and create suspicion in people so that's why they have different websites for products of different niches.