Once you get the rest of the stuff fixed, the easiest solution will be a union query.
Create a totals query that sums the values up to the starting date.
Create a details query for the beginning through ending dates.
Union the two queries and use the Union as the RecordSource for the report.
If you want to display the data in two different fields, you don't need to store the data as two fields. You can use an expression to put the value in one column or the other. Also, I would only show 0 if the amount is 0. The other field should always be null. The zeros are "noise" and I find them disturbing since they have no meaning. You might have a Credit of 0 or a Debit of 0. Using your current layout, you would show 0 in both columns.