There is something most definitely wrong with your car. What you describe sounds weird because the brakes in most modern cars, and I would guess BMWs as well (I pick mine up 7/18), can bring the car safely to a stop even if you are accelerating. [I never understood also all those supposed unintended acceleration claims involving Toyota Camry]. For example, I have tried this with my Porsche and even with the strong engine of the Porsche and low weight of my car, the brakes were strong enough to bring the car to a stop even if pressing the gas (it has no fuel cut off when pressing brakes as some folks that track their cars left foot trail brake a bit in some corners).
Hopefully the dealer will diagnose and fix this for you.