If I understood you right You want when you select the Branch office from first combo the second Combo populate ONLY That branche Loan officers. If So there is a to ways:
1) In InterActiveChange event of the First combo:

SELECT LoanOff
c_filt = "LoanOff.BrancField = '"+this.Value+"'" &&You must Transform the value if it is a numreic
SET FILTER TO &c_filt
thisform.SecondCombo.Requery()

2) You can use put RowSourceType=3 SQL Statement for the second Combo & do something like this in InterActiveChange event of the first Combo:

thisform.SecondCombo.RowSource = "SELECT * FROM LoanOff WHERE LoanOff.BranchField = '"+this.Value+"' INTO CURSOR cLoan"
thisform.SecondCombo.Requery()

HTH