If billing schedules already exist for the parents: Modify the existing billing schedules to generate the correct number of charges and amounts on the correct dates.

OR

Create two separate automatic billing items, one for each parents' preferred payment plan.  For example, if Mom wanted to use a 10-pay plan and Dad wanted to use a 2-pay plan you could set up one automatic billing item with 10 scheduled dates and one with two scheduled dates. Each schedule should be set for their portion of the total amount due.