declare @tab table (col1 char(2))
insert into @tab values
('A1'),
('A2'),
('A3'),
('B1'),
('B2'),
('B3')

SELECT
	A.Col1,
	B.Col1
FROM
(
	SELECT 
		ROW_NUMBER() OVER(PARTITION BY LEFT(Col1,1) ORDER BY Col1) AS RowNum,
		Col1
	FROM
		@tab
	WHERE
		LEFT(Col1,1) = 'A'
) AS A
FULL OUTER JOIN
(
	SELECT 
		ROW_NUMBER() OVER(PARTITION BY LEFT(Col1,1) ORDER BY Col1) AS RowNum,
		Col1
	FROM
		@tab
	WHERE
		LEFT(Col1,1) = 'B'
) AS B
ON
A.RowNum = B.RowNum