CTE's are probably easiest for this, the two queries are I think may be too dissimilar to be merged together:

;WITH Query1 AS (

SELECT

coalesce (cast(DATENAME(MM,hst_date_processed) as varchar(20)), 'Grand Total') as [Months],

SUM(ffh.Net_Sales) AS [Sum Of Net Sales],

SUM(ffh.Net_Trans) AS [Sum Of Net Transactions]

FROM Fact_Financial_History ffh

INNER JOIN Dim_Interchange_Tier_2 i

ON (ffh.hst_prod_code + '-' + ffh.hst_plan_code) = i.Plan_Code

WHERE ffh.hst_date_processed >= @date

AND ffh.hst_prod_code IN ('79','81')

AND ffh.hst_plan_code IN ('002','010','011','015','037','033','021','019','020','030','029')

AND I.Plan_Code <> '79-021'

GROUP BY ffh.hst_date_processed

WITH ROLLUP

--ORDER BY ffh.hst_date_processed desc

),

Query2 AS (

SELECT *

FROM (

SELECT

CASE

WHEN GROUPING(o.FDMSAccountNo_First9) = 1 THEN 'Grand Total'

ELSE CAST(o.FDMSAccountNo_First9 AS VARCHAR(20))

END AS FDMSAccountNo_First9,

o.DBA_Name,

SUM(ffh.Net_Sales) AS TotalofNetAmount,

SUM(ffh.Net_Trans)AS TotalofNetTransactions

FROM Fact_Financial_History ffh

INNER JOIN Dim_Outlet o

ON ffh.hst_merchnum = o.FDMSAccountNo_First9

INNER JOIN dbo.Dim_MCC

ON o.MCC_Code = dbo.Dim_MCC.MCC

WHERE ffh.hst_date_processed > @date

AND ffh.hst_prod_code BETWEEN '79' AND '84'

AND o.MCC_Code = '5968'

GROUP BY

o.FDMSAccountNo_First9,

o.DBA_Name

WITH ROLLUP

) AS temp

WHERE temp.DBA_NAME IS NOT NULL

OR temp.FDMSAccountNo_First9 = 'Grand Total'

)

SELECT WhateverYouWant

FROM Query1

INNER/LEFT OUTER/FULL OUTER JOIN Query2

ON Query2.somecolumns = Query1.somecolumns

ORDER BY SomethingMeaningful