Got a brain teaser for ya. I got 2 domains vincescomputer.com and .biz
The .com is the primary domain but every now and then when you click the Home button on the menu bar it go's to .biz not ,com, this of coarse makes the shopping cart and the Google maps not work.
But then it clears up. Right now it's working fine but for about 30 minutes this evening it did it again.
So I reported it to server support and of coarse they blamed it on Joomla or a setting off. But it only does it once in awhile so that dosen't make sence to me.
In terms of SEO, a parked domain really doesn't help becuase the search engines will still treat them as two separate domains.
So basically, mysite.com/page1 and mysite.biz/page1 will be treated as different domains. in the search engines. So...if you have incoming links to each, that will dilute the pagerank, whereas if you 301 .biz, it will transfer the link juice.