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.
Just try your account and login details for both of them, and so you'll see that they work for both websites. Because I've been playing plinko before with bcgame and then tested the bcpoker when some promos have came. I didn't registered any new account on it and just used the old account of mine and it did worked.