Better to write this query -- the UNION kills the query

Question

Is there a better way to write this qery to avoid the union?
CREATE TABLE EMP
EMP_ID           NUMBER,
LAST_NAME     VARCHAR2(20),
FIRST_NAME     VARCHAR2(20),
MID_NAME     VARCHAR2(20)
CREATE TABLE EMP_NM
EMP_ID          NUMBER,
LAST_NAME     VARCHAR2(20),
FIRST_NAME     VARCHAR2(20),
MID_NAME     VARCHAR2(20)
INSERT INTO EMP
VALUES(
1, 'ANDERSON', 'SCOTT', NULL)
INSERT INTO EMP
VALUES
(2, 'KEVINSKY', 'KEVIN', NULL
INSERT INTO EMP_NM
VALUES(
1, 'ANDERSON', 'SCOTT', NULL)
INSERT INTO EMP_NM
VALUES(
1, 'LEE', 'SCOTT', 'K')
INSERT INTO EMP_NM
VALUES
(2, 'KEVINSKY', 'KEVIN', NULL )
INSERT INTO EMP_NM
VALUES
(2, 'ANDERSON', 'KEVIN', NULL )
SELECT
E.EMP_ID          ,
E.LAST_NAME     ,
E.FIRST_NAME     ,
E.MID_NAME                              FROM          
EMP          E          
WHERE          
E.LAST_NAME          =:LAST_NAME     
UNION
SELECT
E.EMP_ID          ,
E.LAST_NAME     ,
E.FIRST_NAME     ,
E.MID_NAME     
FROM
SELECT     EN.EMP_ID     
FROM          
     EMP_NM     EN
WHERE          
EN.LAST_NAME     =:LAST_NAME                              )          EN1 ,
EMP     E
WHERE
E.EMP_ID     =     EN1.EMP_ID

Answer 1

EXPLAIN PLAN without sort
SELECT STATEMENT Optimizer Mode=CHOOSE          171 K          6717                     
FILTER                                        
TABLE ACCESS FULL     EMP     171 K     5 M     6717                     
TABLE ACCESS BY INDEX ROWID     EMP_NM     1      14      1                     
INDEX RANGE SCAN     IDXEMP_ID     1           3
EXPLIAN PLAN WITH SORT
     Operation     Object Name     Rows     Bytes     Cost     Object Node     In/Out     PStart     PStop
SELECT STATEMENT Optimizer Mode=CHOOSE          171 K          7658                     
SORT ORDER BY          171 K     5 M     7658                     
FILTER                                        
TABLE ACCESS FULL     EMP     171 K     5 M     6717                     
TABLE ACCESS BY INDEX ROWID     EMP_NM     1      14      1                     
INDEX RANGE SCAN     IDXEMP_ID     1           3