I'm sure there is a simple way of doing this but as I'm currently writing a report with a Union query so I will suggest using this method.
Create a basic SQL command which selects just the Initial Date and Initial Payment from the table and then Union with another select statement that only picks Final Date and Final Payment. The fields could be renamed to be generic - Date and payment for example.
You will then only have 2 fields to actually report on. Group by the Date field and choose the monthly option in group settings.
Hope that helps.