Based on the link you provided, it could be something like this:

<launch>

  <!-- Multi-cameras demo with 2 D400 cameras -->

  <!-- Choose visualization -->
  <arg name="rviz"       default="false" />
  <arg name="rtabmapviz" default="true" /> 

  <!-- Cameras -->
  <include file="$(find realsense2_camera)/launch/rs_camera.launch">
    <arg name="camera" value="camera1" />
    <arg name="serial_no" value="1111111111" />
    <arg name="align_depth" value="true" />
  </include>

  <include file="$(find realsense2_camera)/launch/rs_camera.launch">
    <arg name="camera" value="camera2" />
    <arg name="serial_no" value="2222222222" />
    <arg name="align_depth" value="true" />
  </include>

  <!-- Frames: Cameras are placed at 90 degrees, clockwise -->
  <node pkg="tf" type="static_transform_publisher" name="base_to_camera1_tf"
      args="0.0 0.0 0.0 0.0 0.0 0.0 /base_link /camera1_link 100" />
  <node pkg="tf" type="static_transform_publisher" name="base_to_camera2_tf"
      args="-0.1325 -0.1975 0.0 -1.570796327 0.0 0.0 /base_link /camera2_link 100" />

   <!-- sync rgb/depth images per camera -->
   <group ns="camera1">
    <node pkg="nodelet" type="nodelet" name="nodelet_manager1" args="manager"/>
    <node pkg="nodelet" type="nodelet" name="rgbd_sync1" args="load rtabmap_ros/rgbd_sync nodelet_manager1">
      <remap from="rgb/image"       to="color/image_raw"/>
      <remap from="depth/image"     to="aligned_depth_to_color/image_raw"/>
      <remap from="rgb/camera_info" to="color/camera_info"/>
      <param name="approx"          value="false"/>
    </node>
   </group>
   <group ns="camera2">
    <node pkg="nodelet" type="nodelet" name="nodelet_manager2" args="manager"/>
    <node pkg="nodelet" type="nodelet" name="rgbd_sync2" args="load rtabmap_ros/rgbd_sync nodelet_manager2">
      <remap from="rgb/image"       to="color/image_raw"/>
      <remap from="depth/image"     to="aligned_depth_to_color/image_raw"/>
      <remap from="rgb/camera_info" to="color/camera_info"/>
      <param name="approx"          value="false"/>
    </node>
   </group>

  <group ns="rtabmap">

    <!-- Odometry -->
    <node pkg="rtabmap_ros" type="rgbd_odometry" name="rgbd_odometry" output="screen">
      <remap from="rgbd_image0"       to="/camera1/rgbd_image"/>
      <remap from="rgbd_image1"       to="/camera2/rgbd_image"/>

      <param name="subscribe_rgbd"           type="bool"   value="true"/>
      <param name="frame_id"                 type="string" value="base_link"/>
      <param name="rgbd_cameras"             type="int"    value="2"/>
      <param name="Vis/EstimationType"       type="string" value="0"/> <!-- should be 0 for multi-cameras -->
    </node>

    <!-- Visual SLAM (robot side) -->
    <!-- args: "delete_db_on_start" and "udebug" -->
    <node name="rtabmap" pkg="rtabmap_ros" type="rtabmap" output="screen" args="--delete_db_on_start">
      <param name="subscribe_depth"  type="bool"   value="false"/>
      <param name="subscribe_rgbd"   type="bool"   value="true"/>
      <param name="rgbd_cameras"     type="int"    value="2"/>
      <param name="frame_id"         type="string" value="base_link"/>
      <param name="gen_scan"         type="bool"   value="true"/>
      <param name="map_negative_poses_ignored" type="bool"   value="false"/>        <!-- refresh grid map even if we are not moving-->
      <param name="map_negative_scan_empty_ray_tracing" type="bool" value="false"/> <!-- don't fill empty space between the generated scans-->

      <remap from="rgbd_image0"       to="/camera1/rgbd_image"/>
      <remap from="rgbd_image1"       to="/camera2/rgbd_image"/>

      <param name="Grid/FromDepth"     type="string" value="false"/>
      <param name="Vis/EstimationType" type="string" value="0"/> <!-- should be 0 for multi-cameras -->
    </node>

    <!-- Visualisation RTAB-Map -->
    <node if="$(arg rtabmapviz)" pkg="rtabmap_ros" type="rtabmapviz" name="rtabmapviz" args="-d $(find rtabmap_ros)/launch/config/rgbd_gui.ini" output="screen">
      <param name="subscribe_depth"  type="bool"   value="false"/>
      <param name="subscribe_rgbd"   type="bool"   value="true"/>
      <param name="subscribe_odom_info" type="bool"   value="true"/>
      <param name="frame_id"            type="string" value="base_link"/>
      <param name="rgbd_cameras"       type="int"    value="2"/>

      <remap from="rgbd_image0"       to="/camera1/rgbd_image"/>
      <remap from="rgbd_image1"       to="/camera2/rgbd_image"/>
    </node>

  </group>

  <!-- Visualization RVIZ -->
  <node if="$(arg rviz)" pkg="rviz" type="rviz" name="rviz" args="-d $(find rtabmap_ros)/launch/config/rgbd.rviz"/>

</launch>

EDIT: some optimizations could be to attach the rgbd_sync nodelets on the respective realsense nodelet managers instead of creating new nodelet managers as above.