I actually think it is possible to concentrate on different areas of the pectoral muscle.
The main consideration is were teh insertion pointon the humerous is in relation to the origin point of the muscle.
As mentioned you can train one part with out the other, but you can make one area work harder
Some people are just shaped with high insertion points in relation to their origin points, and will cause much more emphisis on the lower pec, if you have a low insertion point there is more muscle above so you get a more balanced looking pec.
This also varies on exercises that alter the two points in relation to each other.
Do this.
have your arm extended downwards at 45 degrees and tense your pec see what area is pumped, now change this so your arm is up at 45 degrees and feel how much more your upper chest is tensed, its not just a re arrangment of the muscle
Doing exercises such as incline db press is great for upper chest, in this movement your pecs become an extension of your deltoid group, you upper chest also plays an important stabilizing role in front db raises, exercises were you can cross your arm over your chest, above your shoulderline are also good.
The pec helps to rotate your upper arm inwards hold your arm strait out in front, palms inner elbow up, tense and feel you chest with your other hand, its flat....
Now roll your hand inwards and feel your chest change shape.
By doing, down wards pushing exercises this will help to isolate the lower pec fibres,
decline db presses, and dips are awsoem exercises for this.
when doing decline db presses, when you get to the top rotate the weights so that your palms are facing at the top, this brings your arms closer by atad which causes a better contraction, and the rotation (different at this angle) also helps to bite in down low.
Anothe rway to test it do a heap of incliune db presses until almost failure, then try a few declines and see, that you can again push the weight up, it feels easier, because that part of the pec is not exhausted. go teh other way if you want.. The pecs are naturally stronger pushing down, because there is generally more muscle mass below the humerol insertion point.. its just mechanics at work.. Also most people use the pec to help lift them selves up a downward push motion, much like your lats, which places more emphisis on the lower pecs..
...I use my clavicles as an idication of my upper chest is too small, if I can see em I train more upper and they go away..