When applying symbology to a feature service in ArcGIS Online, attempting to add a second attribute to the symbology fails if the option to Add Attribute and show the symbology for the two attributes does not appear.
This issue occurs if the source field for the second attribute contains non-numeric values or there are no fields with numeric values in the specific feature to allow the creation of a second attribute. In ArcGIS Online, a second attribute can only be added if the values of the source field are numeric.
A possible workaround is to convert the non-numeric field values to arbitrary numeric values. However, this is only applicable if the non-numeric values can be represented in the arbitrary numeric value format. For example, the field values of 'Yes' can be classified as '1' and the field values of 'No' can be classified as '0'.
To convert the non-numeric field values to arbitrary numeric values, create a new field with the 'integer' field type, and use the Calculate Field operation to populate the field with the desired values. For steps on how to use the Calculate Field operation, refer to ArcGIS Help: Work with fields and ArcGIS Help: Calculate Fields examples for the Calculate Field code samples.
Alternatively, manually input the values after creating the new field.