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I guess you may need to select tree nodes from left tree, and then move them to right tree, if I am correct, there are two solution: 1. Build a multiple selectable tree, get those tree nodes you want to move to the right, build target tree after click >>; 2. Use Tree.setDropHandler(...) to drag-drop tree nodes from left to right column.
If needed, handle nodes from right to left in the same way.