Example: You have a webpage titled exactly like "car forums - automobile chat" and you also want to rank for these keywords. But you ofcourse want to rank separately for each keyword, one is "car forums", the other "automobile chat". If you build links containing both keywords as your anchor text ("car forums - automobile chat"), are you ranking for each keyphrase, or only for the complete "car forums - automobile chat" ? Obviously no one would be searching for the complete keyphrase...
Well, in theory you would probably divide the weight up evenly, but considering we don't have the hard data Google engineers have regarding keyword diluation you can't really say for sure. You better off targeting them on their own rather than together.