Add these snippets to your child theme’s functions.php file or use a plugin like Code snippets. Don’t add code directly to the parent theme’s functions.php file as it will be overridden by updates.

add_action( 'easy_booking_after_update_product_stock_data', 'wceb_share_availability', 10, 4 );

function wceb_share_availability( $item, $product_id, $_product_id, $booked_dates ) {

    // Replace with product IDs you want to share availability
    $share = array( '1', '2', '3' );

    if ( in_array( $_product_id, $share ) ) {

        foreach ( $share as $index => $id ) {

            if ( $_product_id != $id ) { 
                update_post_meta( $id, '_wceb_product_booked_dates', $booked_dates );
            }

        }

    }

}

add_filter( 'easy_booking_get_bookings', 'wceb_get_shared_availability', 10, 1 );

function wceb_get_shared_availability( $bookings ) {

    // Replace with product IDs you want to share availability
    $share = array( '1', '2', '3' );

    if ( ! empty( $bookings ) ) foreach ( $bookings as $booking ) {

        $product_id = $booking['product_id'];

        if ( in_array( $product_id, $share ) ) {

            foreach ( $share as $index => $id ) {

                if ( $product_id != $id ) {

                    $bookings[] = array(
                        'product_id' => $id,
                        'start'      => $booking['start'],
                        'end'        => $booking['end'],
                        'qty'        => $booking['qty']
                    );

                }

            }

        }

    }

    return $bookings;

}