Launching multiple instances of node, not showing up in ros2 node list

edit

I am trying to replicate the ros1 nodelets for pcl passthrough filters. I have written my own passthrough_filter class utilizing similar parameters for clarity.

In my context I would like to run a passthrough filter on the x and z channels of a pointcloud in sequence: original -> x filter -> z filter -> result. Which I do with the Launch file at the end. In the passthrough_filter.cpp I introduce the inputs and outputs as /filters_input and /filters_output such that I can dynamically remap it in the launch file.

The problem is, that running ros2 node list only shows the passthrough_filter_x and not the z, the same is true of ros2 topic list where I only see /velodyne_points/output_x and not output_z. That being said, I can open /velodyne_points output_x and output_z in rviz and see both. If I view output_z then it shows up as a topic, though missing a publisher.

I've included both the launch file and cpp file below. Note, this is in ros2 foxy

EDIT I think this is happening because output_z depends on output_x existing. And launch files don't guarantee execution order. Everything works as expected if passthrough_x is started from the above mentioned launch file and passthrough_z is started from a different launch file. How could I combine these?

LAUNCH FILE:

from ament_index_python.packages import get_package_share_directory
from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
# pointcloud filters
passthrough_x = Node(
    package="pointcloud_filters",
    executable="passthrough_filter_node",
    name="passthrough_filter_x",
    parameters=[
        {"passthrough_filter/field_name": "x"},
        {"passthrough_filter/limit_min": 5.0},
        {"passthrough_filter/limit_max": 30.0},
        {"passthrough_filter/limit_negative": False},
    ],
    remappings=[
        ("/filters_input", "/velodyne_points/transformed"),
        ("/filters_output", "/velodyne_points/output_x"),
    ],
    output="screen",
)

passthrough_z = Node(
    package="pointcloud_filters",
    executable="passthrough_filter_node",
    name="passthrough_filter_z",
    parameters=[
        {"passthrough_filter/field_name": "z"},
        {"passthrough_filter/limit_min": 0.5},
        {"passthrough_filter/limit_max": 3.5},
        {"passthrough_filter/limit_negative": False},
    ],
    remappings=[
        ("/filters_input", "/velodyne_points/output_x"),
        ("/filters_output", "/velodyne_points/output_z"),
    ],
    output="screen",
)
# defining other nodes

ld = LaunchDescription()

ld.add_action(passthrough_x)
ld.add_action(passthrough_z)

# launching other nodes

return ld

CPP FILE

#include "rclcpp/rclcpp.hpp"

#include "pcl_conversions/pcl_conversions.h"
#include "sensor_msgs/msg/point_cloud2.hpp"

#include "pcl/filters/passthrough.h"
#include "pcl/point_types.h"

class PassthroughFilter : public rclcpp::Node {
public:
  PassthroughFilter() : Node("passthrough_filter") {
    pub_ = this->create_publisher<sensor_msgs::msg::PointCloud2>(
        "/filters_output", 1);
    sub_ = this->create_subscription<sensor_msgs::msg::PointCloud2>(
        "/filters_input", 1,
        std::bind(&PassthroughFilter::cloud_callback, this,
                  std::placeholders::_1));

    this->declare_parameter("passthrough_filter/field_name", "x");
    this->declare_parameter("passthrough_filter/limit_min", 0.0);
    this->declare_parameter("passthrough_filter/limit_max", 10.0);
    this->declare_parameter("passthrough_filter/limit_negative", false);

    std::string name =
        this->get_parameter("passthrough_filter/field_name").as_string();
    double limit_min =
        this->get_parameter("passthrough_filter/limit_min").as_double();
    double limit_max =
        this->get_parameter("passthrough_filter/limit_max").as_double();
    bool limit =
        this->get_parameter("passthrough_filter/limit_negative").as_bool();

    RCLCPP_INFO(this->get_logger(),
                "Filter "
                "pararms:\n\t field:\t%s\n\t min:\t%.1f\n\t max:\t%"
                ".1f\n\t negative:\t%s",
                name.c_str(), limit_min, limit_max, limit ? "true" : "false");

    pass_.setFilterFieldName(name);
    pass_.setFilterLimits(limit_min, limit_max);
    pass_.setFilterLimitsNegative(limit);
  }

private:
  void cloud_callback(const sensor_msgs::msg::PointCloud2::SharedPtr msg) {
    pcl::PointCloud<pcl::PointXYZI>::Ptr cloud(
        new pcl::PointCloud<pcl::PointXYZI>);
    pcl::PointCloud<pcl::PointXYZI>::Ptr cloud_filtered(
        new pcl::PointCloud<pcl::PointXYZI>);
    pcl::fromROSMsg(*msg, *cloud);

    pass_.setInputCloud(cloud);
    pass_.filter(*cloud_filtered);

    sensor_msgs::msg::PointCloud2 output;
    pcl::toROSMsg(*cloud_filtered, output ...