One account, twice the way to build up your gamerscore. The Shadowrun and Halo 2 titles are the first to utilize the Live service between Windows Vista-based PCs and Xbox 360.
Not every title will take advantage of the service, nor will every third-party game have achievements. However, it was made clear to us that if they do, you single Live account and Gamertag can have the score built on by both the PC and Xbox 360 games.
You cannot get double achievements, nor will it pick up at the save point if you change games in mid-platform. You will have to start from scratch.