Merging two Select queries with different where statements
Since the two parts are identical except for the WHERE conditions where the first is just a restriction of the second, you can use one query, with the 2nd condition and an altered condition for the 1st sum, i.e. a CASE expression:

SELECT T3.AcctCode, T3.AcctName, 
       SUM(CASE WHEN T0.TaxDate BETWEEN '2014-09-01' AND '2014-09-30'
             THEN T1.Debit - T1.Credit 
             ELSE 0 END)* -1          AS Balance,
       SUM(T1.Debit - T1.Credit)* -1  AS [YTD Balance] 

FROM OJDT T0
  INNER JOIN JDT1 T1 ON T0.[TransId] = T1.[TransId]
  INNER JOIN OACT T2 ON T1.[Account] = T2.[AcctCode]
  INNER JOIN OACT T3 ON T3.[AcctCode] = T2.[FatherNum]
WHERE T0.TaxDate BETWEEN '2014-01-01' AND '2014-09-30'
  AND T3.AcctCode LIKE '611%'
GROUP BY T3.AcctCode, T3.AcctName ;