Generally, when you increase the amount of links to a website it helps search engine optimization. Your rankings will go up, your page rank and likely your traffic. Of course, the important point is, how you get those links. In this video, Matt Cutt’s addresses the question, Should you Link Domains together for SEO? Some webmasters have many domains, so why not use them all to increase rankings for each other!
We’ll, as usual there is a catch. Google does not what you to manipulate their search algorithm by linking too many domains together. Especially by adding a bunch of footer links to your website…
In the video, Cutt’s basically say that the only time you should do this is if you have country top level domains that need to be accessed. He also says, in rare cases it may make sense if you are operating a blog network. But overall, Google is leery of those who link domains together to try to help their SEO.
Video: Should you Link Domains together for SEO?