front end - Dynamically link cell content in Mathematica?

Is it possible to dynamically link the content of two cells? For example, can I set up a Notebook with two text cells that display the same value, and if the user types in one the other updates, etc.? I am familiar with Mathematica's dynamic functionality and at least the key concepts of the FrontEnd, and it seems like I should be able to assign a dynamic symbol to certain parts of a cell and just use that in an expression elsewhere, but I can't figure out how to programmatically access a cell's current content. The best I have been able to do is get a CellObject using Cells[], e.g. where the first cell is a text cell containing "This is a test!", Cells[][[1]] gives

Cell[BoxData[

TemplateBox[{6849,"6849","\"Text\""},
"CellObject"]], "Output",
CellChangeTimes->{{3.5653627710636687`*^9, 3.5653627797706504`*^9}, {
3.5653628101959224`*^9, 3.5653628192779245`*^9}, 3.5653630492614017`*^9}]

but this doesn't include the cell's content.

Answer

See if this does it for you. There are still some issues (see below).

nbA = CreateDocument[{Cell["Notebook A", "Title"],
    ExpressionCell[

     Dynamic[Refresh[
       NotebookWrite[dynamicCellObjB, NotebookRead[dynamicCellObjA]]; 
       dynamicCellObjB = First@Cells[nbB, CellTags -> "DynamicCell"]; 
       "cell to be updated:", TrackedSymbols -> {}, 
       UpdateInterval -> 5]]], 
    Cell["Edit this", "Text", CellTags -> {"DynamicCell"}]}];
dynamicCellObjA = First@Cells[nbA, CellTags -> "DynamicCell"];
nbB = CreateDocument[{Cell["Notebook B", "Title"], 
    Cell["Not this", "Text", CellTags -> {"DynamicCell"}]}];
dynamicCellObjB = First@Cells[nbB, CellTags -> "DynamicCell"];

You need something to trigger the update. Notebooks live in the front end and NotebookObjects, while living in the kernel, are just references to the front end object. Dynamic interactivity requires some variable change or event in the kernel to trigger an update. Here I used Dynamic[Refresh[.., UpdateInterval->5]] to cause an update every five seconds. You can change the update interval as you please. Another key is that the connected cells are identified by a unique cell tag ("DynamicCell" was my choice of name). There are some timing issues. I occasionally got a message on start-up, if a symbol was evaluated before it was defined.

If it seems on the right track, I hope you can adapt it to your notebooks.