Well, I don't know about the technical part, but if I'm not much mistaken, legally purchasing two CD-keys means legally acquiring the license to use two copies of HL/CS/whatever. Thus, I can't imagine why such a transfer would be illegal. You wouldn't damage Valve in any way by doing so, so I don't think they would have a reason to complain.
From a technical point of view, it might work if you copy the HL and CS .gcf (cache) files to the other PC and run a "bare" Steam installer from there... the Steam installer ought to find the cache files and d/l only the latest updates and all other files needed... but this is just an educated guess, so better wait for someone who's more competent in that respect... Nova? He's the Steam guy, hehe...
