SQL Server 2019

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CREATE TABLE dbo.table1(ColumnA char(1), ColumnB int);

INSERT dbo.table1(columnA, columnB) VALUES
('A',          10),
('A',          20),
('B',          15),
('B',          50),
('C',          2 ),
('C',          40);
6 rows affected

-- guaranteed to be ordered by column B:

;WITH src AS 
(
  SELECT columnA, columnB, rn = ROW_NUMBER() 
    OVER (PARTITION BY columnA ORDER BY columnB)
  FROM dbo.table1
)
SELECT columnA, columnB = [1], newcolumn = [2]
FROM src
PIVOT 
(
  MAX(columnB) FOR rn IN ([1],[2])
) AS p;


columnA
columnB
newcolumn


A
10
20

B
15
50

C
2
40


-- arbitrary / "I don't care"

;WITH src AS 
(
  SELECT columnA, columnB, rn = ROW_NUMBER() 
    OVER (PARTITION BY columnA ORDER BY @@SPID)
  FROM dbo.table1
)
SELECT columnA, columnB = [1], newcolumn = [2]
FROM src
PIVOT 
(
  MAX(columnB) FOR rn IN ([1],[2])
) AS p;


columnA
columnB
newcolumn


A
10
20

B
15
50

C
2
40