Some people have done this, but they sometimes try to hide the fact that they own both sites. Thus they register the new domain under a different name or use privacy protection, and they put the new site on a different server.
Some big companies have even created large networks of supportive sites in this way.
Personally I think it's worth it. You might even find you get more traffic from the site you originally created just to support your main site.
Making multiple websites certainly isn't prohibited according to search engine guidelines. Now, if the 'supportive sites' you create are really worthless junk only designed to prop up your main site and that's obvious, and if you use tricky methods like cloaking, etc. then maybe you're going astray of the guidelines.
But, if they're real sites that provide useful information or interaction then you're only making the engines more and more happy by providing them with more food (oops, I mean information).
Thanks aristotle and physics
I have planned to create plain HTML sites with no cloaking or other techniques. These sites would be informative but I would promote my products in them and lead the visitors to my main site.
These sites will have freebies, coupons, links to my main site etc...
And I am planning to add a disclaimer message in the header and footer that these sites are 'owned' or 'sponsored' by my main site. eg; 'XYZ.com' is owned by 'example.com' is it neccessary or does is it have any downsides of adding it.
Note - These sites will be hosted in a different server from that of my primary site.
If done correctly, supportive sites not only support your main site regarding incoming links and traffic, but also support the user. Just think about having a site with a broad topic as main site and you build supportive sites to expand on subtopics.
main site: Photography
supporing sites: landscape, portrait, high definition etc
I have done this in a certain niche where I found subniches and it works quite well.
I will focus on broadening the topics related to my niche.
Thanks for the tip again, It gave me whole lot of ideas.
Makes sense to me from a business sense, so why not.
For the full SEO value, you will want to avoid duplicate content and use a different IP address to host the supporting sites.
In my experience, this is especially helpful if the supporting sites have very descriptive domain names.
The benefit is twofold -- link leverage and you will catch people who may not be attracted to your main site but will be attracted by the descriptive domain name.
When there are multiple trademarks, I create a support site for each. This gives opportunity for content that is not quite on target, but still interesting and relevant.