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Try in your launch file

<group ns="robot1">
<robot launch files etc>
<remap from="cmd" to="/robot2/cmd"/>
</group>

<group ns="robot2">
<robot launch files etc>
</group>

This will put your robots in separate namespaces with ns, and the remap will make the topic cmd be changed to /robot2/cmd the leading slash in front tells ros to ignore your namespace declaration for this remapped topic. Hope that at least gives you a hint!