Geoff, if you mean this picture:

... I believe that that is simply a relatively poor quality image of the two standard Stratocaster three-position slide switches, placed next to each other (slightly offset) and wired in parallel. That is, they each provided parallel paths from the pickups (via neck and middle tone controls where relevant, of course) to the output socket.
If you moved the two switches together (exactly in the same positions), there was no difference between that and the operation of a single switch. If the switch positions were different from each other, then two "selections" were simultaneously making their way to the output socket.
For example:
[*] switch A in neck position, switch B in bridge position - neck and bridge pickups on
[*] A in neck/middle position, B in bridge position - all three pickups on
...and so on and so forth.