A 4iiii dual sided power meter (PRECISION PRO or PRECISION 3+ PRO) measures the power output of both of your legs independently, and will display total power, with the added metric of left/right balance to give you an indication of how each of your legs is contributing to your total power output.
In order to read dual power you must first pair both sides as a dual in the 4iiii App.
NOTE: You will also need to ensure you have enabled a left/right balance data field on the your cycling computer to view the balance data.