Hi,

Unfortunately events can reside only in one calendar at a time.

As an alternative I could suggest you to duplicate the event to the user's personal calendar once the booking order is completed.

This can be achieved with custom hook.

I've attached the required code. 

The code should be placed in your theme or child-theme functions.php file.

When the order containing the event ticket is set as "completed" from your WooCommerce -> Orders menu.

Yes, it's possible with custom filter.

Place the code in your theme or child-theme functions.php file:

add_action('woocommerce_thankyou', function($order_id) {
    if (!$order_id) {
        return;
    }
    $order                         = wc_get_order($order_id);
    $contains_calendar_ticket      = false;
    $contains_non_calendar_product = false;
    $items                         = $order->get_items();
    foreach ($items as $item) {
        $item_data  = $item->get_data();
        $product_id = filter_var($item_data['product_id'], FILTER_VALIDATE_INT);
        if (!$product_id) {
            continue;
        }
        $product = wc_get_product($product_id);
        if ($product && '1' === $product->get_meta('stec_bookable')) {
            $contains_calendar_ticket = true;
        } else {
            $contains_non_calendar_product = true;
        }
    }
    if (true === $contains_calendar_ticket && false == $contains_non_calendar_product) {
        if ($order->get_status() === 'processing') {
            $order->update_status('completed');
        }
    }
}, 10, 1);