Usage of MoveGroupInterface with several planning groups

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I'd like to ask for help understanding some aspects of the MoveGroupInterface in MoveIt2. Especially the usage of the interface while having a robot or scene with several planning / joint groups.

All the questions are referencing the run_move_group demo nodes, the node and the interface.

I implemented the example for my own robot and it is kinda working, but im wondering about the usage:

My robot has 3 planning groups at the moment, which i'd like to keep. Two of them are individual robots, and the last one is just of all the joints in my planning scene, so the compound group doesnt know anything about the EEFs or anything beside the joints.

Im getting some instances of the Interface:

moveit::planning_interface::MoveGroupInterface group_robot1(move_group_node, "master");
moveit::planning_interface::MoveGroupInterface group_robot2(move_group_node, "slave");
moveit::planning_interface::MoveGroupInterface group_both_robots(move_group_node, "both");

Where the first two can return their EEF and the last one cant, which i would expect. And all of them can return the possible planning groups, but i dont see how one should use that.

I also can see 4 nodes of my move_group_int getting created after i start this interface demo, which is kind of strange, since i want to access the already running move_group node. Is this intended?