As was said, it's best to do most of your braking before the turn, but you can use both brakes in a turn with a little care. Like a motorcycle, the bike will tend to straighten up when you brake hard while leaning. You can practice adjusting to this, and practicing all your techniques will allow them to become second nature.
As a fatty, I use both brakes hard when I really need to scrub off speed and I shift my weight way back. I also descend in the drops most of the time and weight the outside pedal in tight turns.