At the first look, it should be something similar to:
SELECT RM.RepId, UM.UserName AS Sender, UM.UserName AS Receiver
FROM Requestmaster AS RM
LEFT JOIN Usermaster AS UM ON RM.SenderId = UM.UserId
LEFT JOIN Usermaster AS UM1 ON RM.ReceiverId = UM.UserId
[EDIT]
Here is working sample:
DECLARE @UserMaster TABLE (UserId INT IDENTITY(1,1), UserName VARCHAR(30))
INSERT INTO @UserMaster (UserName)
VALUES('user a'),('user b'),('user c'),('user d')
DECLARE @RequestMaster TABLE (RepId INT IDENTITY(1,1), SenderId INT, Recieverid INT)
INSERT INTO @RequestMaster ( SenderId, Recieverid)
VALUES(1, 2), (3,1)
SELECT RM.RepId, UM.UserName AS Sender, (SELECT UserName FROM @UserMaster WHERE UserId = RM.Recieverid ) AS Reciever
FROM @RequestMaster AS RM
LEFT JOIN @UserMaster AS UM ON RM.SenderId = UM.UserId
Result:
1 user a user b
2 user c user a
[/EDIT]