I can draw a minimap from a gamestate (e.g. world) via in my Minimap Gamestates render function.
public void render(float tpf) {
super.render(tpf);
WorldGameState wgst = (WorldGameState) GameStateManager.getInstance().getChild("world");
Node wRootNode = wgst.getRootNode();
// very important since world scene should be drawn in minimap
tRenderer.render(wRootNode, fakeTex);
}
But how could I acomplish the drawing of a minimap from more than one gamestates rootNode?
maybe you could create a MiniMapGameState which has references to the other gamestates ?
I did it that way
Is this a good way or exists better ones? At least it works 
in construction of monitor / minimap gamestate
// prepare scene for texture
combineNode = new ArrayList<Spatial>();
WorldGameState wgst = (WorldGameState) GameStateManager.getInstance().getChild("world");
ObjectGameState ogst = (ObjectGameState) GameStateManager.getInstance().getChild("object");
combineNode.add(wgst.getRootNode());
combineNode.add(ogst.getRootNode());
list = new ArrayList();
list.add(fakeTex);
and in render I did
tRenderer.render(combineNode, list);