Basically what the title says.
I want to do this, because I want to copy my Mac game to my Windows PC, without re-downloading it.
The executables and libraries may be different between Mac and Windows, but I would expect the assets to be the same in most cases. And the assets are what take up the most space.
I would copy the files over and then open Steam, go to the Library, right-click on the game and hit Properties, go to the Local files tab, and hit "Verify integrity of game cache". That should verify the assets and allow you to download the platform-specific files.
I've only done this Windows -> Windows with partial backups of games, but hopefully this works! Edit: David Freitas confirms in a comment on the question that you can actually do a regular Steam backup and restore without copying files manually, awesome.