SQL Server 2017

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CREATE TABLE T(
   UserName VARCHAR(50),
   CompanyName VARCHAR(50)
);

INSERT INTO T VALUES ('UserOne','CompanyOne');  
INSERT INTO T VALUES ('UserOne','CompanyTwo');  
INSERT INTO T VALUES ('UserTwo','CompanyOne');

DECLARE @tables AS NVARCHAR(MAX),
        @query  AS NVARCHAR(MAX),
        @col AS NVARCHAR(MAX);
        
WITH CTE AS (
	SELECT *,ROW_NUMBER() OVER(PARTITION BY UserName ORDER BY UserName) rn
	FROM T 
),CTE1 AS(
	SELECT MIN(rn) smallRN,MAX(rn) bigRN,UserName
	FROM CTE 
    group by UserName
	UNION ALL
	SELECT smallRN+1,bigRN,UserName
	FROM CTE1
	WHERE smallRN < bigRN
)
SELECT @col = STUFF((SELECT distinct ', MAX(CASE WHEN rn = '+CAST(smallRN AS VARCHAR(50)) +' then CompanyName end) as CompanyName' +CAST(smallRN AS VARCHAR(50)) 
            FROM CTE1
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)') 
        ,1,1,'')

set @query = '
WITH CTE AS (
	SELECT *,ROW_NUMBER() OVER(PARTITION BY UserName ORDER BY UserName) rn
	FROM T 
)
SELECT UserName,'+@col+' 
FROM CTE t1 
GROUP BY UserName'

EXEC (@query)

UserName	CompanyName1	CompanyName2
UserOne	CompanyOne	CompanyTwo
UserTwo	CompanyOne	null

Warning: Null value is eliminated by an aggregate or other SET operation.