The difference between the ECG1 and ECG2 is due to the different positioning of the electrodes on the belt. Both use the central sensor (front left location) as a reference, but the other electrode responsible for the potential difference has different positions, as shown in the image below.
ECG2 basically works as a backup for ECG1, but if you are analyzing both at the same time, there may be a slight difference between them due to this positioning.