config.yml

        frequency:
            - [ P2D, 5 ]
            - [ T13M, 20 ]

<?php
$root
    ->children()
        ->arrayNode('frequency')
            ->prototype('array')
                ->validation()
                    ->ifTrue(function ($node) { return count($node) !== 2; })
                    ->thenInvalid('Frequency should have arrays with length 2')
                ->end()
            ->end()
        ->end() // frequency
    ->end();