just start up the download on steam in OSX, this will build the folders for L4D2 under ~/library/application support/steam/steamapps/
and in bootcamp, go to your program files/steam/steam apps and just copy the l4d2 folder into the same directory as above in osx.
Start up steam and it will begin downloading. It will stil show 0% or 1% or wherever you were at but it will skip over files you already have in the folder, so it'll go from 0%-100% but a lot faster than it looks. I was thrown off by this for awhile, but you can tell when it shows total download 3GB but you've gone from 100MB to 9GB after a couple hours.. meaning it's skipped a bunch of files saving you time.
After you're done, make sure to verify integrity of game cache.. right click the game, properties, data or something tab and then verify. Good luck!