Text Box Validation on a Web Form (Help with a Custom Validation)

Question

I am having diffuculty with a validation.

I have two text boxes: one for last name and one for first name.

I need to validate the last name text box for data ONLY when the first name box contains data.

Basically, if someone inputs a first name, they must also input a last name. The user does not have to enter a first and last name, but if they enter one they must enter the other. The boxes can contain any alphabetic value, I'm just validating for presenece of data.