I think Matt Cutts once said that the dash is something like a separator for G, so if it's kw1-kw2, G will see it like kw1 kw2; but if it's kw1kw2 G will see it like kw1kw2.
But, in my opinion, that's rubbish. Now, I'm not saying Matt is wrong, I'm just saying it was probably an old video I watched.
Because, when I search for something, like, say, automobiles, one of the results that comes out is .automobilemag.com, and, as you can see, automobile is bolded, which means the SE understood it was part of the KW.
So, in my opinion, in terms of SE's, it doesn't matter. In terms of brandability and ease of remembrance, definitely go for kw1kw2.
But, if you're doing an autoblog, it doesn't really matter.