This is fairly easy actually, but it will sound more complicated in step-by-step text. Since you have a RAW file save one version as a JPEG at normal exposure levels, and a second version at -2.
Open both files normally. Pick the file with the smallest portion you want to use (assume its the -2 version for now, and all you want is the facial portion), and click on that image to select it as the active image. CTRL-A to select all, CTRL-C to copy, then click on the other (normal exposure) image to select it as the active image, and CTRL-V to paste as a second layer into that image. You can also do this with right-click commands as well as the CTRL keystrokes. You will now see a second layer in the layers palette, and if the second layer is selected as the active layer, the entire image will look like the -2 exposure image. then select Layers/new mask layer/hide all, you'll see a black mask box to the right in the layer icon, and now your image will look like the normal exposure image. Now select the brush tool, and paint white over where you want the -2 exposure image to be selected, you'll see it in place of the other image everywhere you paint white. to get a more gradual transition you can paint gray around the white areas, and you can adjust the opacity of all changes with the slider in the layers palette. You can also experiment with other blending modes, but for this application normal blending mode is probably as good as anything. If you paint too much white, just switch to black and paint out the excess white to get the normal exposure portions to reveal in the composite image.