SQL> WITH test AS (SELECT 'Rajesh' first_name, 'Singh' last_name FROM DUAL
  2                UNION ALL
  3                SELECT 'Suresh', 'Pandey' FROM DUAL
  4                UNION ALL
  5                SELECT 'Raj Kumar', 'Pratap' FROM DUAL)
  6  SELECT first_name
  7         || CASE
  8               WHEN first_name = 'Rajesh' AND last_name = 'Singh' THEN ' '
  9               WHEN first_name = 'Suresh' AND last_name = 'Pandey' THEN '  '
 10            END
 11         || last_name
 12            result
 13    FROM test;

RESULT
-----------------
Rajesh Singh
Suresh  Pandey
Raj KumarPratap

SQL>