Does forwarding one domain to another domain 'mess' up google's indexing?
Maybe I haven't explained my concern properly but I have read that when you use two different domains on the same site - once you've navigated to that site, all of your pages use that domain. When googlebot crawls both domains, it will treat each as as separate site. When it finds that both sites have the same pages, it will penalize both of them for being spammy.
Is that correct?
I have a domain that isn't being used but it is 'related' to another domain that I am using. If I point the unused domain to the 'running' website will that mess google up?
It depends on how you're doing it. If you have a simple page in the site that redirects you to another domain it should work fine.
On the other hand, if you use aliases, where the browser address always stays at whichever domain you came in on, i could easily see that splitting up the stuff. I would imagine physical redirects would be best.
I don't do it though. eg: i own a site www.e-referrer.com that gets over 21 million page views per month. I also have ereferrer.com ereferer.com and e-referer.com (just incase people mis spell it) all configured as aliases. I have noticed that it looks like my stats go down since i've done this. The reason I mention the amount of traffic is to let you know that it's a real trend, not just a few hits i'm talking about.
Show your reciprocal links on your website. eReferrer