Zend_Input_Filter - dependent fields on each other - how to?

Question

I'm developing a web application using Zend Framework 1.12.3.

I've got 2 fields that need validation using Zend_Filter_Input, let's call these fields "foo" and "bar".

While they are not required, and "foo" could exist on a standalone basis, if "bar" is given "foo" also must exist (i.e. if "bar" is defined then "foo" is required).

Do you know how to accomplish this? Thanks