How to limit employees from enrolling into two plan types concurrently via Event

We use On-Demand Event Maintenance for enrolling employees into benefits with HCM 9.2 Ben Admin module. While we have two plan types of Child Rider Insurance, our policy does not allow employees enrolling those two plans concurrently.

We were able to disallow enrolling one of the two plan type if an employee has already enrolled into the other plan type by setting up Component PeopleCode, but it only works when we manually enter the enrollment. Since Event is processed by COBOL program, we need to implement something to limit the enrollment.

What is the best way to configure system so that we do not have to manually watch Child Rider enrollment? I appreciate your inputs!