I made a selection of David's face and copy/pasted it onto Young Lady In A Boat. I scaled it down while next to the girl's face for comparison. In this case it was important for the eyes and the chin to line up. I put his layer over her face and then the fun began. Lots of fiddling around with getting the coloring as close to hers as I could. I used a mask for some of it, especially around the edges and the hair hanging in her face. I did some outright painting on David's face and some on the girl's hand which is in the top layer - touch ups. I painted over, at a low opacity, David's eyebrows with her hair color and his mouth with the color of her mouth. I picked up colors with the eyedropper from the girl's face. I did not use the selection I made of the girl's hair that you can see in the Layers palette.
After I was done, I pushed the greys/middle triangle towards the white in Levels.