I can attest to that. I have registered on the main bcgame and was able to participate in some poker tournaments conducted before with bcpoker. I just used the same account of mine and didn't have to register to bcpoker because they share the same database. So both registration complements each other and a user can login his credentials to both websites.
Well, that's quite good to hear! I've not used the same username and password of BC.GAME on BC poker and instead registered a new account at BC poker and for me that worked as well without any issues. However, I would say that on BC poker a player has more controls like changing one's avatar etc. I know such a thing might be possible on BC.GAME as well but I didn't try that on main site of BC.GAME and tried it on BC Poker instead.