I have to apologize. I not only got it backwards, but emphasized my mistake. I set the exposure for the SHADOWS (Zone 2 or 3) with the flashing light off, then turn the flashing light on and adjust it to give Zone 8 0r 9 in the highlights with both lights on. The highlights thus have an amount of light about equal to the threshold. The reading in the shadows is practically unchanged because the flashing illumination is a small percentage of the illumination from the enlarger in the shadows, and the Zone readings are logarithmic. The shape of the paper's H&D curve is changed so that the highlights are flattened, so it is not good to make major adjustments by flashing. This is true whether it is done in two separate exposures or in one combined exposure.