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Vertical align 2 jface TreeViewers to specific element.
treeViewer.addSelectionChangedListener(new ISelectionChangedListener() {
  @Override
  public void selectionChanged(SelectionChangedEvent arg0) {
     TreeViewer mirrorTree = (treeViewer == treeVwrSource ? treeVwrTarget : treeVwrSource);
     // find position of selected element (element x) in treeViewer
     // set position of element x in mirrorTree, it is already selected.
  }
});

Tree tree1 = ...;
Tree tree2 = ...;

int topIndex = tree1.indexOf(tree1.getTopItem());
tree2.setTopItem(tree2.getItem(topIndex));

viewer.setSelection(otherViewer.getSelection)