I would do it with a pair of macrobutton fields. Each field would be in a bookmark. The macros would operate to delete the text in the other bookmark and then unlink the remaining field. You can set the document so that a single click runs the macro.

For it to be more advanced, or if you want more text than can be in a macrobutton prompt, you could use AutoText. See the checkbox Add-In template on my Downloads page for an example of a macrobutton that inserts AutoText. In that case, the AutoText is another MacroButton, but it could be anything that AutoText can handle.