Why are there two columns on a PT chart, and how are they used?
The two columns on the PT chart give the liquid and vapor pressures at the listed temperatures. Single component refrigerants and azeotropic blends have bubble points and dew points equal to each other, and we simply call this the boiling point. When there is only one column on the PT chart, low glide blends would have very similar numbers in the two columns, and often the PT chart will only have one column as well for them.
How a two-column PT chart is used is straightforward. Most times you're interested in knowing the saturated temperature of the refrigerant at the system pressure, so you can compare it to a measurement you're making on the system (for example, to check a superheat or subcool setting). Simply keep track of the condition of the refrigerant where you're measuring, and cross-reference the same side of the PT chart.
Superheat measurements check the line temperature of superheated refrigerant vapor coming out of the evaporator versus the saturated vapor temperature, so you would use the vapor side of the PT chart.
Subcool measurements check the temperature of subcooled liquid refrigerant coming out of the condenser versus the saturated liquid temperature, so you would use the liquid side of the PT chart.