If both reviewers customised the data extraction form independently, the resulting data would not be consistent. Covidence sets up the data extraction function in such a way that both reviewers must agree on the data to be extracted so that both reviewers are extracting the same set of data.
The reviewer assigned as the first reviewer can make edits to the customisable elements data extraction form: intervention characteristics, interventions, baseline characteristics, and outcomes.
You can change the person assigned as first reviewer by clicking "Manage Reviewers" underneath the reference, which will open the menu shown below. Use the drop-down menus to assign reviewers, then click "save".