Try either multi-table update syntax
UPDATE config t1 JOIN config t2 ON t1.config_name = 'name1' AND t2.config_name = 'name2' SET t1.config_value = 'value', t2.config_value = 'value2';
Here is SQLFiddle demo
or conditional update
UPDATE config SET config_value = CASE config_name WHEN 'name1' THEN 'value' WHEN 'name2' THEN 'value2' ELSE config_value END WHERE config_name IN('name1', 'name2');
Here is SQLFiddle demo