Single or multiple domains?

I run a motorsport discussion board,
the main address is
Motorsport Forums

But I also own www.rallyforum.com

For a long time I've had the main site at motorsportforums.com but I've also had rallyforum.com trigger a different forum style and had all the branding etc on the site as rallyforum.com

My question is, is it better from an SEO etc point of view to concentrate on one address, or is having rallyforum.com as a seperate domain (but with the same content apart from header logs etc) useful at all?

Having multiple domains will only hurt your rankings as you'll create duplicate url

You should just park any extra domains on the main domain, and make sure you have your 'not this, make it this' redirect up, like as offered in [How to] Setup www.yourdomain.com or yourdomain.com (www vs. non-www) - With .htaccess 301 redirects - vBulletin SEO Forums