FacetWP - use a dummy facet to populate 2 data sources

functions.php

<?php

// Add the following to your (child) theme's functions.php

add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'region_partner' == $params['facet_name'] ) {
        $params['facet_name'] = 'region_dropdown'; // trick FacetWP into saving values into the other facet
    }
    return $params;
}, 10, 2 );