select the head with one of Photoshop's selection tools
When doing this, I find it quite helpful to create an intermediate image with a transparent background that contains only the head to be pasted into the second image. First I paste the head onto this transparent background, scale it, rotate it or skew it to make it fit into the other image, adjust the lighting, then select it.
To be able to paste the head seemlessly into the second image (and without aliasing effects), I make the border of the selection soft (⌥⌘D); my Photoshop has a german interface, so I am not sure about the english localization, this command must be something like "selection -> modify selection -> soft border".
I would not bother to get the lighting and color excactly right in Photoshop, that is far easier with the "dodge" and "burn" brushes and color adjustments in Aperture.
To do this completely in Aperture, you would have first to combine both images into one (for example by creating a book page), then use the retouche->clone tool to stamp the head from one image to the other. I use this occasionally to stamp a face into a flower bud or for similar effects. However, for this to work you need a plain background and nearly circular shapes to be pasted.
Léonie