Planning creates an entry for every new text in the relational DB. It does not create an entry if it finds an exact match and uses the exiting string's id as the value.
So if cell1's value is "Text1", after running a calc script to copy cell1 to cell2, cell2 is now "Text1" as well. There is only one entry in the table. Now if you go and edit either one of them to "Text1 Modified", a new entry gets added to the table for that cell. If you go back to "Text1", no new entry will be created.
So in short if cell B changes, Cell A value will not change.
Goog explanation. I'm adding to it
You are right about Planning creating an entry in relational. It adds the text and adds an ID.
This ID gets stored in Essbase and it has an internal mechanism to match the ID with Text.
So what happens when you perform a copy (DATACOPY) this copies the data in Essbase.
So now CellA and Cell B has same number and they will point it to the text. When you change the text no new entry and so both shows the same number.
A simple data copy won't help you. You don't have much of an option here (other than playing around with the tables and adding new entry there and send new data value to Essbase.