Validating dependent fields

What would be best way to validate the following example?

People can sign up for a race. Every team consists of a driver and a navigator. During the initial sign up they can fill-in an whole bunch of data but only 2 are required.

These would be the name and email from the driver or from the navigator.

Is there an easy way to check this with the built in validators by stacking them? Or is it possible to write a validator that can check other fields?

Any help would be welcome.

Thanks.