It feels like there are too many TOP 5's in this, but maybe not :)...

Also, it's not clear what the constraints are; e.g., can different rows in the same or different table have the same dates? It is ALWAYS better to post real code, rather than (over?) simplified table definitions.

CREATE TABLE tbl_class_a
(
u_name varchar(20),
dt_joined date
);
CREATE TABLE tbl_class_b
(
u_name varchar(20),
dt_joined date
);

INSERT tbl_class_a VALUES ( 'a1', CURRENT DATE );
INSERT tbl_class_a VALUES ( 'a2', CURRENT DATE + 1 );
INSERT tbl_class_a VALUES ( 'a3', CURRENT DATE + 2 );
INSERT tbl_class_b VALUES ( 'b1', CURRENT DATE );
INSERT tbl_class_b VALUES ( 'b2', CURRENT DATE + 1 );
INSERT tbl_class_b VALUES ( 'b3', CURRENT DATE + 2 );
INSERT tbl_class_b VALUES ( 'b4', CURRENT DATE + 3 );
INSERT tbl_class_b VALUES ( 'b5', CURRENT DATE + 4 );
INSERT tbl_class_b VALUES ( 'b6', CURRENT DATE + 5 );
COMMIT;

SELECT TOP 5 *
  FROM ( SELECT TOP 5 u_name, dt_joined
           FROM ( SELECT * FROM ( SELECT TOP 5 'a' AS source, * 
                                    FROM tbl_class_a  
                                   ORDER BY dt_joined ) AS a
                  UNION ALL
                  SELECT * FROM ( SELECT TOP 5 'b' AS source, *  
                                    FROM tbl_class_b  
                                   ORDER BY dt_joined ) AS b ) AS ab
          ORDER BY source, dt_joined ) AS top5ab
 ORDER BY dt_joined, u_name;

u_name,dt_joined
'a1','2011-08-01'
'b1','2011-08-01'
'a2','2011-08-02'
'b2','2011-08-02'
'a3','2011-08-03'

try simple query like that:

select TOP 5 dt_joined, u_name 
    from 
    (
    SELECT TOP 5 dt_joined, u_name from tbl_a order by dt_joined
    UNION ALL 
    SELECT TOP 5 dt_joined, u_name from tbl_b order by dt_joined
    )
    order by dt_joined

What about this?

select TOP 5 dt_joined, u_name 
from 
(
SELECT TOP 5 dt_joined, u_name from tbl_class_a 
UNION ALL 
SELECT TOP 5 dt_joined, u_name from tbl_class_b 
) as bt
order by dt_joined asc