This post answers the following questions
- Does it help in SEO to host the same content in two domains?
- Is it a good idea to host two copies of your site?
- Can I use two domain names?
Well the answer is no, and absolutely not. If you host duplicate content the end result might send both your domains to bottom of the search results. This creates a duplicate content issue, so google will think at least one of the site is copying or mirroring the content. And you will never get double the traffic. And your new domain will never see the top of the search rankings.
So if you are planning to migrate to a new domain, then set a permanent 301 redirect in the old domain for at least 3 months before retiring the old domain. If your audience come mainly through Search engines this should be fine, but if they come through other site links, then you might need to change those links or keep the old site's redirect on.
You can find how to set up a permanent 301 redirect here