Let other forms submit in checkout page.

custom-checkout.php

<?php

/**
 * Let Core checkout page have another forms and submit them. 
 *
 * 
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function custom_checkout() {
	if( function_exists( 'pmpro_is_checkout' ) && pmpro_is_checkout() ) {
	?>
	<script>
		jQuery(document).ready(function($) {
			jQuery('form').submit(function(){
				if( $(this).attr('id') != 'pmpro_btn-submit' ) {
					jQuery('#pmpro_btn-submit').removeAttr('disabled');
					jQuery('#pmpro_processing_message').css('visibility', 'hidden');
				}
			});
		});
	</script>

<?php
	}
}
add_action('wp_head', 'custom_checkout');