As sometimes turns out to be the case, I'll answer my own question.
From Rob & Dave's Air-cooled VW Pages
(http://www.vw-resource.com/master_cylin ... l#switches
), we have this:Brake Light Switches
Concerning the brake light switches on the master cylinder, Rob wrote -- The master cylinder has two switches in it (I'm assuming the "modern" Mexican bugs have similar master cylinders to the older German beetles). One switch for rear brakes and one switch for the front brakes, so if one brake line fails you still have half your brakes and your stop light still work.
The switches are in parallel - either or both will work the stop lights They screw into the master cylinder, and are easy enough to replace. You just pull back the rubber protector boot, pull off the wires, unscrew the switches and replace them. You will probably have to bleed the brake lines afterwards - some air will probably get into the lines. A spongey feel to the brake pedal is a sign of air in the brake lines.
Certainly for safety redundancy, one switch for rear brakes and one switch for the front brakes. Who knew?
Here is a helpful conceptual wiring diagram for the switches, wired in parallel (as Weaponer