Here it is (squint until you can see the shape of the "vase" - or negative space stand out): 

and here's the vase by itself - see if you can't switch between seeing the two separate faces then seeing the unique shape of the the face: 

Here's a face-off comparison of  both hair shapes (contrasted in Photoshop), the two lumps in Arnie's hair are numbered off to the right):

Both eyelids are outlined here (roughly)

Davis does

-Can you identify a pupil or iris? (the colored part of the eye) or is your brain "imagining" one there?

I'd have to say Arnold

and what's the shape of those contours that make  the wrinkles around the eyes?

Arnold has more of the angular "Crow's feet" sort of configuration where as Davis has more of wrinkle - especially under the lower lid.

Can you see the lighter shapes of the actual skin of the eyelids? (Arnold has a rounder shape in the upper lid compared to Davis's. Davis has a more rectangular shape outlined. (Both Gov's upper lids are outlined by the eyebrows above and the actual margin of the upper eyelid below)

Sorry about the repetition...See picture above - it's obvious

(You really can make out an iris and pupil in Davis's picture if you look close :-) Also here I've outlined the "Crow's feet" in both (see the  picture just above to see an unmarked view of the crow's feet in both) :

Obvious here:

Here you can see the apron of the upper lip highlighted in purple (I don't know how it got be purple, it just got that way :-); and it's obvious the owner of the greater span is Gov Davis: 

A better question might be to ask "what's the shape of light and shadow areas in and around the dimples, and how do dimples and dimple contours heighten a positive  or "happy" look" (I quote "happy" because there's just a pinch of a suggestion of smirk in Arnold's smile. Is that just me or do you see it too?)

Notice how Arnold's dimples (in this picture) are really two highlights with an hour-glass shaped or barbell-like (or even telephone-handle shaped) area of shadow. (Yes you have to use your imagination a little).

Here, the highlight shapes are outlined in yellow, the shadow in blue. Squint to make this more obvious. Shadow and highlight shapes, though we're very nearly unconscious of them under most circumstances (except for now while we're consciously pointing them out), are as unique to each person as are the actual features. Observe and capture those shapes and you'll add an immense amount of realism to your drawings. You can make this kind of observation about all shadow and highlight shapes. 

This fleshier tip, gives the illusion that the margin of the upper lip in a side view moves down as you move from corner of mouth to front most tip (like the tip of the lip just under the nose). And the truth is, it actually slopes down away from horizontal as you move left to right.

Davis has a prominent dip of the tip of the upper lip. This is actually the termination of the fleshy side of "Cupid's Bow". (Open up your e-sourcebook, open up the Search function  and type in "Cupid's Bow" for more info.)

• Adam's apple in each? 
• In front? 
• Behind? 
• How far behind? 
• Where would this line fall in  comparison to the backmost edge of the nostrils? 
• The corner of the mouth? 
• How far to the tip of the nose? 
• To the notch along the nose at the same level as the eyes?

Use the following picture to answer these. Measure right on the screen - ie do a sighting. The differences are subtle - but are real enough to exaggerate! (The eye notices these differences, but measuring  and comparing nails it down when it's not clear.) Compare these distances face to face too (i.e. Arnold to Gov/ Davis).

Arnold's certainly are more prominent. Higher? Hard to tell. Look close though - what do the shadows under the cheeks tell you about the shape of the cheeks? Arnold's have much more of a comma shape suggested. See that? Look close, squint, back away from ur computer until this is clear  :-)

I'll let you do this...ok, I'll bail you out one more time :-) 

Hoo man, that's way too close! (Photoshop is evil...the macho man in me is being threatened...and yes, I'm straight.) It's a little easier to make out shadows that define the cheeks in Arnold, but back away (from your screen) and see if you can't make something out of Davis's. Look at how these areas were managed (as shadow shapes), in the caricature:

They're pretty well defined in the caricature above. Now see if you can't find them in this picture:

Compare the angles the white of the shirt collars make with horizontal outline of the overall photograph...then with the vertical edges of the photo...then with the jaw.

Use the picture above - you can do it!

Ahaa! Another negative space exercise you say? I can't fool you!  You are amazing.  By just observing the negative space. Arnold's got more slope away from vertical than does Gray. You see it right? No? Do a sighting! (Don't know what sighting is? Go back to the Archives and look in the Flash Quick reference section - or use the search function in your e-sourcebook.

The two blue lines on each face are the exact same length. This proportion is fairly accurate. Seems like just a little extra nudging out past the ears.

Can you envision the general outline  of the hair? (I'm repeating myself again.) Squint your eyes until you can see the hair as one shape - be especially observant of the hair border at the temples and around the ears. Where are the highlights in both hair-dos? The Shadows? How does color of the hair influence both shadow and highlight?

Here's the highlights in the hair - (can you spot the shadows? - squint until you see the darkest areas clearly):

Who's skin is darker? Or is the contrast of Gov. Davis's white head of hair making his skin appear more tan than it is? 

I think Arnold is darker in both respects - skin and hair.

Look at each ear. Who's seems more tilted away from the vertical? Now look at the detail in both ears. What can you say about the 3 drawable parts: 1) the overall shape, 2) the anti-helix, and 3) the shadow area)? You'll probably want to use the bigger picture for this.

By looking at the sport coats and ties, can you make any speculation about the rotation of the upper body in each? That is, is one person facing the other more straight on, more square to the other?

If these two were wolves they wouldn't be wearing suits :-)

Asking Specific Questions

So, after going through with all that, do you kind of have an idea how you might draw a caricature of these two? After you approach a picture or a live subject asking the same kinds of specific questions, your brain will go to work and I guarantee, you'll come up with your own hilarious ideas how to exaggerate these two. If you feel like sending in your exaggeration I'll happily post it for you - and it'll be included in the next version of the book.  :-)