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I run multiple ecommerce stores in one particular market. I've been considering merging them all together and using a Single sign-on and allowing users to swap between websites. Each site is unique in their own way and are already ranking well on their own. But the goal is to merge them altogether to create a better user-flow. An example of what I'm trying to do is what Zurb.com does (http://zurb.com/apps). They have all of their different products but they're under different domains. Another example is http://www.envato.com/sites and all of their brands to their sites. Will this negatively impact SEO efforts across the board or will we eventually benefit from merging them. Also, is there a correct way to do this. For example; Should I make one site the "parent website" and then create sub-directories of the other websites and work on the DNS to point to the right locations. I'm not the technical person on our team but I do lead the marketing and I can't find the right answer for this question.