Using scope_identity() to get ID "during" insertion of row?
Do exactly as Aaron said, or if you are afraid of nulls, or don't want to use COALESCE, perhaps you could do something along these lines:

USE tempdb;
GO
-- Create the table
CREATE TABLE dbo.T
(
    IdentityID INT NOT NULL CONSTRAINT PK_T PRIMARY KEY CLUSTERED IDENTITY(1,1)
    , CopyOfIdentityID INT NULL 
);
GO
-- create a trigger to copy the values from column a to column b
CREATE TRIGGER T_Trigger ON T
AFTER INSERT
AS 
    UPDATE dbo.T SET CopyOfIdentityID = i.IdentityID FROM dbo.T INNER JOIN inserted i ON t.IdentityID = i.IdentityID;   
GO
-- insert some test values (this would typically be some other piece of code, perhaps
-- a stored proc or something.
INSERT INTO dbo.T DEFAULT VALUES;
-- you could return the SCOPE_IDENTITY() here for use in the update below.
SELECT SCOPE_IDENTITY();
-- show the row prior to changing the b column to some other value
SELECT *
FROM dbo.T;
-- update the b column to some other value.
DECLARE @SomeOtherID INT;
SET @SomeOtherID = 2;
UPDATE dbo.T SET CopyOfIdentityID = @SomeOtherID WHERE T.IdentityID = 1;
-- show the row with the updated value.
SELECT *
FROM dbo.T;


Results:



Simple, and doesn't rely on anything fancy.