For example, set the due date to be the last Thursday of the month; or the second Monday, or first Friday, etc. I personally have a weekly task that I complete every Thursday, but before each next month arrives, I have a monthly task that is directly related to and a part of this weekly task.
Task B must be completed on the same day as the last Task A of the month and both tasks are closely related;
I complete Task A every Thursday;
I complete Task B at the end of each month whenever the last occurrence of A occurs in that month.