3815460 - How do I make the second field in a connected list required only when a user populates the first field?

I have 2 fields on the Report Header and the Expense Entry forms. They are a connected list using fields Custom3 and Custom4. They are not required for all users, but if a user selects an item from the Custom3 list they are then required to select an item from the Custom4 list.

The client has a connected list and the fields are not required on the form. If the user selects an item from the first list field they are required to select an item from the second field. , KBA , CON-EXP-ADM-FOF , Forms and Fields , CON-EXP-ADM-AUR , Audit Rules , Problem