Whatever an agent tells you, the COC prohibit "impossible" bookings and US employs software to ferret this out. If US is using the AA software which is more recently updated and better, you stand a greater chance of being caught.

The question is what will happen if caught. If lucky, you get a call. if sort of unlucky, US cancels the "correct" ticket. If really unlucky, both are auto-cancelled.

Bottom line is that there is a risk. Whether you take the risk depends on your risk tolerance.