It's been a while but my recollection from dealing with street centerlines is that both resulting geometries would follow the direction of the origin polyline. A polyline has a FromPoint and ToPoint which gives you an idea of the direction it is going. If you need to know which is upstream for the other, check the new segments ToPoint & FromPoint... if lineA.FromPoint = LineB.ToPoint then Line B is 'upstream' (according to the original feature's direction).