Way Way Way back Google penalized a well know company that I believe were selling gifts and also owned 5+ others sites that basically took the first half of page 1 for major keywords.
Google's terms and conditions suggest not doing this, especially if you are already targeting the same niche or similar with similar content. If asked by Google would you be able to explain it that would not sound like you were trying to game the results?
HOWEVER! So many sites are getting penalized so easily these days that you would have to be a fool not to have backups. This is the very bad side to slow and sometimes impossible recoveries that many site owners have been reduced to owning multiple sites because anything can happen.
