functions.php

<?php

/**
 * Callback for SearchWP engine choice dropdown. Swaps out engine config for
 * chosen engine at runtime.
 * 
 * @param $settings
 * @param $query
 *
 * @return mixed
 */
function my_searchwp_engine_dropdown_handler( $settings, $query ) {
	$engines = SWP()->settings['engines'];
	$selected_engine = isset( $_GET['swpengine'] ) ? esc_attr( $_GET['swpengine'] ) : 'default';

	if ( isset( $engines[ $selected_engine ] ) ) {
		$settings = $engines[ $selected_engine ];
	}

	return $settings;
}

add_filter( 'searchwp_engine_settings_default', 'my_searchwp_engine_dropdown_handler', 10, 2 );