[villagetinker]

OP, the box you are looking for is the one where the wires from the breaker box goes to. The 'best way' IMHO to determine this will be to turn OFF the breaker. Go to one of the switch boxes, disconnect the all of BLACK wires and LABEL where they were installed. Make sure that and you are clear of the wires and turn on the breaker, then using a voltmeter determine which wire (there should only be ONE) is energized. If you get more than one black wire energized you are in the wrong box. Repeat above in the other box. Once you get to the correct box with ONE and only ONE energized black wire, your can go and install the new switch.
Now have stated the above, I strongly suggest you get at a minimum a handy man knowledgeable in this area or a licensed electrician to do your project.
Please be careful.