An age old argument for many of us here is whether or not to have more than one site for anything.
Arguments for one site are fairly important, i.e. concentrate all your effort on one site, spend time on that one and build the importance of it etc.. however i wanted to lay down an argument for a second/more sites..
Im going to list some reasons why i think its an idea, but please feel free to either add to the list, agree with me or disagree with me or any points for any reason.
IMHO, i think 2 or multiple sites mean..
1) You are not putting all your eggs in one basket e. i.e. if one got penalised/lost many rankings.
2) You can see if a loss in visitors is down to your site or trend in the market, providing they are on different servers and not detectable as one entity.
3) You can keep one as your main and experiment with the other i.e. paid links or riskier SEO strategy in general etc..
4) You can be more targeted in your advertising and hopefully gain a higher click through rate
5) Less internal pages 'can' mean more weight put on internal pages
6) See how conversions on one site compare to the other/s.
7) Possibly higher rankings for targeted key terms on each site due to factors such as incoming links using your preferred keyword/domain in the anchor text.
8) The more of your web pages rank in one search, the less competitors there are to compete with you on that page.
9) One site might rank well in one search engine and others in another.
10) lower bounce rate as site is more targeted to key terms user is using.