T-SQL (SS2K8) :: Trying To SUM Row With Current Date To Row With Last Month Date

I am trying to SUM a column of ActivityDebit with current Calendar_Month to a Column of Trial_Balance_Debit from Last Calendar_Month. I am providing Temp Table code as well as fake data.

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IF OBJECT_ID('TempDB..#MyTrialBalance','U') IS NOT NULL
DROP TABLE #MyTrialBalance
CREATE TABLE #MyTrialBalance (
[Trial_Balance_ID] [int] IDENTITY(1,1) PRIMARY KEY CLUSTERED NOT NULL,
[FISCALYEAR] [smallint] NULL,

[Code] ....

Here is my Query I am trying but not working. I cant figure out how to doo the dateadd for correct column.

SELECT A.Trial_Balance_ID,A.ACTIVITYDEBIT --SUM(A.ACTIVITYDEBIT + B.Last_Trail_Balance_Debit) AS New_TB
FROM
(SELECT [Trial_Balance_ID], [Calendar_Month],[ACTIVITYDEBIT]
FROM Mytrialbalance
WHERE actindx='48397' AND ACTIVITYDEBIT='820439.78000'
)A INNER JOIN
(SELECT [Trial_Balance_ID],DATEADD(MM, -1,Calendar_Month)AS Last_Month
FROM Mytrialbalance) B ON B.Trial_Balance_ID=A.Trial_Balance_ID