The Kurus apparently didn't have a well defined rule for determining successors to the throne. Yayati made Puru his heir even though he wasn't the eldest of his sons. Bharata, the great patriarch and the founder of Bharata Dynasty, made Bhumanyu ascend to his throne, denying all his nine sons, because he considerd them incompetent. Shantanu became king ahead of his elder brothers Devapi (who was denied because he had a disease and who later went out to become a hermit) and Bahlika (who went to rule his maternal kingdom). And there probably are more such examples. In all these cases the common aspect was that the present ruler decided who would become his heir.
Now in the case of the Pandavas and Dhartarashtras, Duroydhana, truly was the heir of choice of the then present ruler, Dhritarashtra. On the other hand, the Pandavas, too were sons of the previous ruler. If Dhritashtra was appointed regent, then the legal heir of the kingdom would have been one of Pandu's sons. If Pandu, when he left Hastinapura had denounced the throne completely for himself and his heirs (like Bhishma did), the legal heir would have been Duryodhana. Unfortunately the Mahabharata does not provide an answer to this question, which makes one feel that there wasn't any decision made prior to Pandu's death.
Under such circumstances a division of the kingdom seems to be the only reasonable solution. And that was eventually done, by giving the Pandavas the woodlands of Khandavaprastha`and making Duryodhana the heir apparent to Hastinapura. But such a settlement proved to be too unstable to maintain, with proud and powerful warriors getting hungry for more on both sides of the border, and with the long history of rivalry between the two groups. Within a few years, with the dice game and Draupadi's harassment, the war became inevitable.