Put a sticker on it marked "clean". Need a plate? Take it out of the "clean" dishwasher. Got a dirty plate? Put it into the other dishwasher. Other dishwasher full? Turn it on and move the "clean" sticker to that one.
Double buffering with dishwashers turns them into magic cupboards. I don't know why most kitchens only come with one.
The sticker/magnet doesn't work.
What works is maintaining the invariant "if the door is locked, the dishes inside are clean". If you open a locked door, you either empty the dishwasher or relock the door.
One is enough if it's a Fisher & Paykel DishDrawer. Two independent dishwashers in one: http://en.wikipedia.org/wiki/Drawer_dishwasher
For storing things it probably wouldn't be big enough for more than a couple of people though. Also, magnet is better than sticker.
What about items that you don't use fast enough? They'd stay left in the clean machine and you'd have to manually put them away before it turns into a "dirty" machine.
Maybe you really need more than 2 machines?