I'm guessing the switch would have to know the impedance of the speakers it is connected to. When it's feeding both in parallel you're right without adding anything two 8ohm speakers would be seen as 4. But the switch could add an inline 4ohm resistor before the paralleling to bring the total back up to 8. I guess this is not a vary good solution cause of the losses. But off the top of my head I can't think of a non-active solution that inccurs no losses.