Multiple planning scenes

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I understand that you want to plan a motion with a different ("future") PlanningScene while another motion is being planned or executed using another ("current") PlanningScene. I don't think this is readily available in the way you imagine it, but you could maintain two different move_groups with their respective PlanningScene. It would make sense for one of them to use the PlanningSceneMonitor to maintain the current scene and the other move_group to be used only for planning, in a PlanningScene that you define and maintain manually.

In general, you can maintain multiple PlanningScenes and use them to plan in alternative future states (it's done this way in the MoveIt Task Constructor, which you might want to look at), but this is advanced usage, so it is not available through the MoveGroupInterface or PlanningSceneInterface.