I agree that those instructions (page 10 of the manual) are not clear. However, bearing in mind that the front smoke unit is a dual chamber model and the funnel into it that is underneath the stacks is connected to both stacks but only one side/chamber of the unit, I think that the instructions are to put 10-20 drops down one stack only:
Now, that amount of smoke fluid won't be sufficient for this unit because the second chamber (whistle smoke) is separated from the first by a divider and the smoke fluid must flow under it and up to contact the resistor in the second chamber. People have different views about how much smoke fluid is too much or too little. I would normally charge a new smoke unit of this kind with at least 30-40 drops but added slowly, say in two or three batches, to let it soak in. Which stack you choose to put it down makes no difference as they are connected at the funnel.
