Usage
Hoverboard port can be set as a "port" parameter in the node namespace. Check hoverboard.launch and 4x4.launch for examples of 2WD and 4WD configurations. 4WD config is purely for illustration on how to use several ports, but was not tested with two hoverboards.
roslaunch hoverboard_driver hoverboard.launch
Use any tool (keyboard_teleop, rqt) to send speed commands to hoverboard_velocity_controller/cmd_vel.
In 4x4 configuration, if you want to control both axis with the same cmd_vel, you can apply remapping as following:

<launch>
    <group ns="front">
        <param name="port" type="str" value="/dev/ttyTHS1"/>
        <remap from="/front/hoverboard_velocity_controller/cmd_vel" to="/cmd_vel"/>
        <include file="$(find hoverboard_driver)/launch/hoverboard.launch" />
    </group>
    <group ns="rear">
        <param name="port" type="str" value="/dev/ttyTHS2"/>
        <remap from="/rear/hoverboard_velocity_controller/cmd_vel" to="/cmd_vel"/>
        <include file="$(find hoverboard_driver)/launch/hoverboard.launch" />
    </group>
</launch>