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In terms of pure SEO benefit, it is not very helpful. It also can get you penalized, if it's abused/the majority of your backlinks. However, there can be very legitimate reasons to do and can bring you traffic. For example, if you sell widget X, and people who buy that widget tend to buy widget Y, then mutual linking could help you both. Perhaps something like, one person sells iPhones and you sell iPhone cases. Ruben

Not sure I understand what you mean.  Maybe I did a poor job of explaining. The sites are different enough where they merit having their own domains but they do share a common theme.  I only manage one of these domains but can get the others to link to us.  I'm trying to determine how they should link to us.