Why not add both sources to the same AutoCompleteStringCollection then:
DataClasses1DataContext db = new DataClasses1DataContext();
var drt = db.Franchises.Select(x => x.DistrubitorCompany).ToArray<string>()
AutoCompleteStringCollection acscName = new AutoCompleteStringCollection();
acscName.AddRange(drt);
#endregion Distrubitor Company
var drta = db.Deposites.Select(x => x.DepositeName).ToArray<string>();
acscName.AddRange(drta);
txtDepositedTo.AutoCompleteCustomSource = acscName;
txtDepositedTo.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
txtDepositedTo.AutoCompleteSource = AutoCompleteSource.CustomSource;
Regards,
Manfred