Wearing a regular bra during exercise: No Go!
A regular bra is not suitable for sports, because it doesn't offer the necessary support and is not designed for taking on too much activity. These straps would cut into the shoulders during intensive workouts and the underwire, seams or decorative extras could cause uncomfortable pressure points.
High-quality sports bras are made of materials that wick away sweat. A regular bra worn under a sports bra would block that function and absorb the moisture instead of transporting it away from the skin.
But why would busty women even think of wearing two bras for workouts?