MYSQL ORDER BY CASE Issue

I have a database like this:

-------------------------------------------------------------------
|   id_one   |   id_two   |   timestamp_one   |   timestamp_two   |
-------------------------------------------------------------------
|     27     |     35     |        9:30       |        NULL       |
-------------------------------------------------------------------
|     35     |     27     |        NULL       |        9:35       |
-------------------------------------------------------------------
|     27     |     35     |        9:34       |        NULL       |
-------------------------------------------------------------------
|     35     |     27     |        NULL       |        9:33       |
-------------------------------------------------------------------

I need pull all 4 rows

ORDER BY 'timestamp_one' if 'id_one'=27 or 
ORDER BY 'timestamp_two' if 'id_one'=27

This is the statement I have now:

SELECT * FROM tablename 
WHERE id_one=27 OR id_two=27 
ORDER BY 
  CASE WHEN id_one=27 THEN timestamp_one END DESC, 
  CASE WHEN id_two=27 THEN timestamp_two END DESC

This works good in that is outputs this:

-------------------------------------------------------------------
|   id_one   |   id_two   |   timestamp_one   |   timestamp_two   |
-------------------------------------------------------------------
|     27     |     35     |        9:30       |        NULL       |
-------------------------------------------------------------------
|     27     |     35     |        9:34       |        NULL       |
-------------------------------------------------------------------
|     35     |     27     |        NULL       |        9:33       |
-------------------------------------------------------------------
|     35     |     27     |        NULL    开发者_如何学C   |        9:35       |
-------------------------------------------------------------------

But I need to two timestamp columns to order like they are one so it would order like this:

-------------------------------------------------------------------
|   id_one   |   id_two   |   timestamp_one   |   timestamp_two   |
-------------------------------------------------------------------
|     27     |     35     |        9:30       |        NULL       |
-------------------------------------------------------------------
|     35     |     27     |        NULL       |        9:33       |
-------------------------------------------------------------------
|     27     |     35     |        9:34       |        NULL       |
-------------------------------------------------------------------
|     35     |     27     |        NULL       |        9:35       |
-------------------------------------------------------------------

I hope this makes sense. Essentially, I am trying to have two ORDER BY columns that are specific to a WHERE condition. Then once the correct ORDER BY column is chosen for that row, it orders the ROWS by the timestamp as a whole.

SELECT id_one, id_two, timestamp_one, timestamp_two      
FROM tablename 
WHERE id_one = 27 
    OR id_two = 27 
ORDER BY 
    CASE 
        WHEN id_one=27 THEN timestamp_one 
        WHEN id_two=27 THEN timestamp_two 
    END DESC 

SELECT 
   id_one,
   id_two,
   (CASE 
      WHEN id_one=27 THEN timestamp_one
      WHEN id_two=27 THEN timestamp_two    
   END) as timestamp
FROM tablename
ORDER BY timestamp DESC

SELECT * FROM tablename 
WHERE id_one=27 OR id_two=27 
ORDER BY id ASC, timestamp_one DESC