IF HART works at analog O/P if so then how is it possible one pair of wire can work at AI and AO at the same time

Field Instrumentation
1

How is it possible if one pair of wire works at AI and AO at the same time if the HART works at analog O/P

2

HART is a 4-20mA analog signal with digital superimposed on the same wires. It uses the same concept as caller ID uses