What has been commonly referred to (in the United States, at least) as ‘clustering’ parishes is usually an application of c. 526 §1, which allows the diocesan bishop, in the case of a shortage of available clergy, to make the same priest the parochus of more than one parish at the same time. The text of the canon itself is quite clear that this is a provision for what is still seen by the legislator as an exceptional case:
A pastor is to have the parochial care of only one parish; nevertheless, because of a lack of priests or other circumstances, the care of several neighboring parishes can be entrusted to the same pastor.
This was not envisioned by either the Code Commission or the legislator as becoming the norm, but as an accommodation that would be available should circumstances require it. It is no secret, however, that circumstances have required it on a vast scale in the years since the promulgation of the Code, to the point that it is difficult to consider this arrangement as even nominally exceptional anymore. It should be noted that the canon specified neighboring parishes, for obvious practical reasons.
It is worth considering what is happening juridically in such an arrangement. The provision of c. 526 §1 has provided an exception to the general rule of c. 152: “Two or more incompatible offices, that is, offices which together cannot be fulfilled at the same time by the same person, are not to be conferred upon one person.” The parochial care of two (or more) distinct parishes simultaneously could be considered incompatible to the extent that it is difficult or impossible for one man to adequately fulfill the pastoral obligations toward each parish. Yet, as Antonio S. Sánchez-Gil points out, while it is easy and logical to think of “the pastoral care of various parishes in practice as a single task that the parish priest must perform, in a strictly juridical analysis it seems preferable to consider said task as a plurality of offices.” 
The same author continues:
In fact, pastoral care is the spiritual end for which the office of parish priest is established (cf. c. 145), and it is not identified stricto sensu with him. Consequently, when the Code speaks of entrusting the care of several parishes to the same priest, it seems that it must be understood that this is done through the canonical provision to the office of parish priest in each case (cf. cc. 146, 986 §1). Moreover, it is easier this way to distinguish, right from the act of provision, the possible particularities that, according to the pastoral needs of the case, could affect the parishes in question and to consider repercussions in the corresponding office: such offices can be entrusted at the same time or at chronologically separate moments; it can be established that there be a single act of taking of possession or several (cf. c. 527); a parish priest can be appointed for an indefinite time in one parish or for a determinate time in the rest (cf. c. 522); his office can lapse in one parish and remain preserved in others (cf. c. 538); etc. On the other hand, the consideration of a plurality of offices emphasizes more the autonomy of the entrusted parishes: in effect, each parish conserves unaltered the configuration that it had before the appointment of a common parish priest, and it will be opportune to maintain, therefore, in the majority of cases, a juridical representation separate from each one of them (cf. c. 532). 
There is a lot of detail to consider in that quoted text, but in summary, c. 526 §1 makes it possible for one priest to cover several assignments simultaneously, without — and this is what makes this in many ways the simplest option available — making any juridical modification to the canonical configuration of any of the parishes thus provided with a parish priest. The map of the diocese remains unchanged; it is only the pastoral assignment chart which is altered.
 Antonio S. Sánchez-Gil, in ExComm, Vol. II/2, p. 1312 (emphasis in original).