When controls for both Loco brakes & train brakes are different in a loco there is no question of limitation about usage of both brakes simultaneously or using as per choice.
Now usage of both brakes differs with LPs experience.
I don't know what discussion is taking place elsewhere but there is no concept of Dual Brakes being regarded as Train brakes & Loco brakes.
Dual...
more... brakes always refers to Vacuum brakes + Air brakes.