What is the best way to do a conditional when in Joi?

I would like to do:

if (a === 'Value1' || b === 'Value1') {
  // Check the length is less than 300
}

And have this so far...

{
  a: Joi.string(),
  b: Joi.string(),
  c: Joi.string()
    .when('a', {
      is: Joi.valid('Value1'),
      then: Joi.string().max(300),
    })
    .when('b', {
      is: Joi.valid('Value1'),
      then: Joi.string().max(300),
    })
    .label('C label')
}