[Brendon]

If you simply want the 2 domains to exist, but people to end up at one of them, use a 301 (it's very important that it is a 301, NOT a 302) redirect. Do NOT duplicate the site on other domains.

If you want 2 actual sites in the index, build a brand new, genuinely unique site for the 2nd domain. Don't be tempted to skimp, and just reskin the old content, you'll get hammered for it. Feel free to put big "visit our main domain" buttons on the second domain, but make it somewhere worth going in it's own right too