If we understand the function of a muscle, that muscle certainly can be trained. Indeed, if a muscle has a function, performing that function is training the muscle. So the question is whether there exists such a thing as ‘inner’ and ‘outer’ pectoralis muscles, and if so, what their function is.
To be clear, there is strictly no such thing as inner and outer pectoralis muscles. That is, the lateral and medial portions of the muscles are comprised of the same fascicles, and hence the tension in those muscle fibres is consistent across their length during contraction. There are no medial and lateral fibres because the fibres of the muscle do not run vertically. So by definition, we cannot train the lateral and medial portion of the muscle separately.
However, it is possible that the terms ‘inner’ and ‘outer’ are misnomers for the anatomical terms deep and superficial, in which case we would be referring to the pectoralis minor and pectoralis major, respectively. The terms ‘lower’, ‘middle’, and ‘upper’ refer to the general alignment of the muscle fibres of the pectoralis major, with the latter term normally being made in reference to the clavicular portion of the muscle, (illustrated below).
Although the two muscles are geographically adjacent, they have broadly different functions. The pectoralis major acts principally as a prime mover, and is dominant in climbing, pressing, and throwing movements. The pectoralis minor, by contrast, primarily acts as a stabiliser, depressing and protracting the scapula. Thus, the action of the former invariably involves the latter.
For bodybuilding, a knowledge of the precise action and function of a muscle can help us isolate it in training. And whilst it is not possible to separate these muscles entirely—their lines-of-pull are too closely associated—we can reduce the function of one in order to focus on the other.
The pectoralis major has its origin at the anterior surface of the medial half of the clavicle (clavicular head, or ‘upper’ pectorals) and the anterior surface of the sternum and superior costal cartilages (sternocostal head, or ‘middle’ and ‘lower’ pectorals). It then inserts at the lateral lip of the bicipital groove of the humerus. This means that if we adduct and internally rotate the humerus, and if we depress the shoulder girdle, the pectoralis major will approach a state similar to that of active insufficiency, being unable to produce force because of the length-tension relationship. (It cannot strictly be called active insufficiency, since the pectoralis major is not a multi-joint muscle.) Thus, this condition will allow greater contribution from the pectoralis minor, which inserts on the coracoid process of the scapula, and whose action is to depress and protract the shoulder girdle. Pre-fatiguing the pectoralis major will further place emphasis on the minor.
Since the pectoralis minor is not functionally a prime mover, no gross functional movement will target it specifically. However, if we are creative, we can devise exercises that might help. Shoulder depression repetitions from a arms-down, pronated-grip position on the bar would be one way to do this.
I hope that gives you a starting point.