To do it all under one account, you could register machinery.com as your account's "primary" domain, and digger.com etc. as "add-on" domains. They would be published under public_html/digger/ etc. as machinery.com/digger. I think you should be able to get to the "digger" page either by digger.com or machinery.com/digger.

Since search engines will notice that there is duplicate content between the two ways to get to a given page, they may ding you for that (or just list one way or the other in their results). I'll have to leave that discussion for someone better versed in SEO, as to whether that's a real problem or not. You may end up wanting to make machinery.com's links to http://www.digger.com instead of just /digger, so that search engines treat them as totally separate entities (I think).

I definitely add my "what they said" to the above posts.

Remember that the Search Engines (SE) will see each page individually and maybe a tiny teeny bit of relationship between pages in the same domain.  If an SE sees the exact same page on crane.com as it does dozer.com then they will "think" - hey, no one wants to see both of these... which is best? and choose one and the other will be on page 4-bazillion.  You never want to have multiple sites with the exact same pages.  You don't have to be worried about one paragraph that appears in multiple places, but much more than that should probably be avoided.

Additionally, links to dozer.com from puppies.com will not be worth as much as a link from crane.com since it's a related site.  Don't put a lot of links from one to the other all over the place, but if you ever have a place where it makes sense for the page at crane.com to mention dozer.com, go ahead and link over.  Basically, the rule is that if it will help a visitor, do it, but don't do it to scam the search engines.

The best thing is to figure out who knows what they are talking about and who doesn't.  There is a lot of bad information out there about SEO so don't take everything said as truth unless the source can be trusted.  Here's something that may help - If the people at the site are relying heavily on the browser toolbar pagerank value, it's probably not too good.  I have a site that has a reported PR of 2 but it's #1 for the main search phrases... go figure. 

There's a lot to consider and to learn about SEO so take it slow and ask questions!

Coincidentally, I was just researching this same scenario.  Client has multiple domains they want pointed to the same website.

According to this thread at Google Webmaster forums the correct method would be a primary domain, and other URLs should give a 310 http response.


What I have done on my account is have one folder in my public_html oritinally setup with one domain.  Then all the other domains are made additional add-ons and pointed to the same folder.  The response I get when i go to one of those URLs is a 301 and it displays the same site.  

What I'm still looking for is whether or not to submit the sitemap multiple times under each domain name.

Everyone has there own opinion,  but if you have the time, optimize each niche (keywords) on a seperate site, then link them all together to the relevant pages, a great trick for this is to also use 2.0 sites, this way u have authority that Google love, link them all together and then point a link again from each on to a different page on your site.