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However, to completely cross-link all your sites, and especially in a mechanical way throughout all the footers, seems a bit dicey to me. It might work out OK, but I personally wouldn't do it. Even if it isn't a problem in the moment, it just feels like the kind of thing that might blow up in the future.
I've also seen sites of common ownership both appear in the serps without apparent problems.
I wouldn't be doing it if the site content wasn't similar, though. That would look like spamming. But in my case, someone who is interested in red widgets might be interested in blue widgets, so it makes sense to link them.