Repeating the contents of one cell in another, in “real time”

Question

How could I create two Input cells, such that when the first is evaluated, the contents of the first are copied to the second? So for example, there would be two empty cells - let's call them A and B - and when the user enters f[x_]:=x^2+4 in A, nothing happens. When that same user evaluates cell A, then the contents of cell B are updated to be f[x_]:=x^2+4. There is a similar question here, but I don't need to be that low-level about things, and I also don't want automatic refreshing; only when cell A has been evaluated.

Answer 1

Why not use CellEpilog? :

CellPrint /@
  {ExpressionCell[ , "Input",
    CellTags -> "CellA",
    Background -> LightGray,
    CellEpilog :> Module[{contA},

      contA = NotebookRead[Cells[CellTags -> "CellA"]] /.
        {Cell[b_, "Input", ___], ___} :> b;
      NotebookFind[EvaluationNotebook[], "CellB", Next, CellTags];
      SelectionMove[EvaluationNotebook[], All, CellContents];
      NotebookWrite[EvaluationNotebook[], contA]
      ]],
   ExpressionCell[, "Input", CellTags -> "CellB",
    Background -> LightOrange]
   };