Add some divs to create 2 columns for Billing Address and Payment Information Fields at membership checkout.

pmpro_two_column_checkout_sections.php

<?php
/**
 * Add some divs to create 2 columns for Billing Address and Payment Information Fields at membership checkout.
 */

function pmpro_checkout_columns_divs_after_user_fields() {
	echo '<div class="row"><div class="medium-6 columns">';
}
add_action( 'pmpro_checkout_after_user_fields', 'pmpro_checkout_columns_divs_after_user_fields', 1 );

function pmpro_checkout_columns_divs_after_billing_fields() {
	echo '</div><div class="medium-6 columns">';
}
add_action( 'pmpro_checkout_after_billing_fields', 'pmpro_checkout_columns_divs_after_billing_fields', 1 );

function pmpro_checkout_columns_divs_after_payment_information_fields() {
	echo '</div></div>';
}
add_action( 'pmpro_checkout_after_payment_information_fields', 'pmpro_checkout_columns_divs_after_payment_information_fields', 99 );