Guru Nanak did not pass the succession to either of his sons. He gave it to a disciple, Lehna, whom he renamed Angad, and the Sikh tradition records the tests by which the choice was made and reads the whole episode as establishing that the office rests on demonstrated devotion rather than on descent.
The doctrinal difference usually named is the householder ideal. Nanak taught a life of work, family and remembrance within ordinary society, explicitly rejecting withdrawal, and the Guru Granth Sahib contains sustained criticism of ascetic display. An order of celibate renunciants founded by his own elder son therefore stands in an unresolved relation to the tradition: recognisably descended from it, and organised around a practice it declined to adopt.
The relationship was not simply hostile. Sikh accounts record meetings between Sri Chand and later Gurus conducted with respect on both sides, and a widely repeated tradition holds that Guru Hargobind's eldest son, Baba Gurditta, was given to the Udasi line as its head, which would make the two lines formally connected as well as parallel.
Those accounts should be read for what they are. The chronology attached to Sri Chand in traditional sources gives him a lifespan of well over a century, spanning the tenure of several Gurus, and the meetings are recorded in janamsakhi and later hagiographical material rather than in documents of their own time. They tell you what the tradition holds about the relation between the two lines, which is worth knowing, and they are not evidence of the kind that would fix a date.