Let me explain in more detail for future readers. The Access navigation form uses only ONE subform control. That means that only ONE subform can ever be loaded at one time so you cannot ever refer to subformB from subformA since they will never be loaded at the same time.
theDBguy's suggestion to use Tempvars solves the problem but it is a two step process. subformA populates the TempVar with the value but subformB has to pick up the value from the TempVar and do something with it.