the help shows a list of lists, whereas an exported run shows a semi-colon delimited string. maybe they are interchangeable
one field
arcpy.management.CalculateGeometryAttributes("pnts", "xs POINT_X", '', '', None, "SAME_AS_INPUT")
two fields
arcpy.management.CalculateGeometryAttributes("pnts", "xs POINT_X;ys POINT_Y", '', '', None, "SAME_AS_INPUT")
try [["new_field", "AREA_GEODESIC"]] since the help shows a list enclosed in a list for multiple parameters, perhaps it needs the enclosing list even if one is chosen
... sort of retired...