I second the suggestion for using a redirect instead–especially since you mentioned search engines in your post.
Here are examples of the difference. In either case, the content you see is whatever you have at domain2.com and domain1.com is the one that either redirects or mirrors.
Redirecting will always show the user domain2.com in the address bar.
Mirroring will show whichever one is typed in, but still display the content of domain2.com.
With mirroring, you’ll see the same thing either way. That means search engines will as well, so you could run the risk of a duplicate content penalty if both names get indexed (which only takes one link). This can even be the case by using both www and non-www versions of the domain.
With redirects, you’re safe, and it doesn’t matter which people link to… if you’re worried about PR.
An example of when to use a mirror instead would be if you’re going to dynamically load different content based on the domain that’s requested, but don’t really need to fully host the domains individually.
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