Is there a way (using Dreamweavers behaviors) to make certain a user enters data in a field_#2 if they enter data in field_#1 ?

For example, I have a form with the fields:

Volume: (already with edited with behaviors to accept numeric only)

Units: radio buttons that consist of gallons (0-1), quarts (0-4), pints (0-8)

How can I add a behavior that will check to make sure at least one of the units is checked IF the user enters a volume? The volume fields are mostly optional fields on the form.