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Normally redirects are 301 meaning supposedly they are permanent, now you can use a 302 redirect telling the search engine that the redirect is not permanent and it shouldn't update its registry with the contents of site B however if you want to keep the rankings on site A whilst ranking site B you can't really look at a redirect as a shortcut. If you look at it in the eyes of Google you redirect a site only to undo the redirect a little later down the line it would look sneaky. What you could do however is get some links on site A to site B and some of the link juice would be transferred to site B this way (though not as much as a redirect) and work on a SEO strategy this way. On the subject of domain name aside from ease of user remembering the name the URL doesn't really have an impact on SERPS. Unfortunately there isn't really a shortcut to ranking both sites as a redirect. Hope some info above was helpful.