The fallout between the two of them would probably happen as Cao Wei begins collapsing in the northern sections of the empire, assuming a post-Guan Yu scenario. The last Cao loyalists would probably remain a substantial power on the northern outskirts, such as in Hebei, as the breakdown in relationship would inevitably come about with the division of the vast amount of lands falling under their control and as conflicts ensue over overlapping land claims. A later resolution between them had divided the Chinese state between them in theory, but it is unlikely that it would have remained. The Central Plains and Jing province would be the most contentious area, particularly as a Shu Han without parts of Jingzhou would be unable to effectively govern much of the new territory in China without being incredibly vulnerable. Sun Wu is in a better position, but they have had the least amount of success translating their naval power into land power, and their armies rarely had any hope of projection beyond the Chang Jiang river valleys. With jagged growth in Shu Han, and staggered growth in Sun Wu, and the Cao Wei collapsing and condensing to the far north, the very strategic situation that made the Three Kingdoms era partially stable would fall apart.
If this is pre-Guan Yu, Sun Wu would probably attack Shu Han as the Liu's start to consolidate control in the central plains, potentially resulting in a northern and southern dynasty situation that may become a stable arrangement, or a one which either the north or the south wins.