As I interpret this, you want to use two of the old 7.2V backup battery (14.4V total) to run the DS and router, and you want to make a
voltage divider with a resistor.
I have two better ideas for you:
First, you could go to RadioShack, a hobby store, or numerous internet sites and buy a larger NiMH rechargeable battery pack and charger. A ten-cell pack will have a nominal voltage of 12 volts.
Second, you could purchase a voltage regulator chip (such as the
LM317T) from a supplier like
DigiKey or
Mouser, and rig up a 12V regulator circuit. This would be far more reliable, even as the batteries discharge or if their is a current fluctuation in either the router or DS.