I think that the significance of the ending cannot be overstated here.  This would be one of the first locations where I would move towards in terms of assessing the meaning of the barriers that exist towards Fielding and Aziz in being friends.  The most natural barrier towards their friendship is the obvious political climate in which an Indian person and a British person cannot be friends.  This is reflected in their political discussion in the jungles, one in which Aziz is staunch in his "Quit India" perspective and one in which Fielding is quite driven to argue that India will fragment without the British.  This political difference is a significant one, and proves to be a barrier towards them being friends, or even being able to be allied with one another.  When the ground opens up causing a different path for each horse to take, I think that it becomes a significant point that is symbolically representative of the political rift that seems impossible to overcome.

I would also state that there is another reason in the end that helps to bring out why their friendship is impossible on another level.  There is a spiritual exploration and a sense of spiritual nebulousness that Fielding is willing to explore and Aziz is not.  Fielding wants to know more about Hinduism and the elements that are within it.  He seeks a sincere understanding as to why Stella and Ralph are drawn towards it.  There is a spiritual exploration that Fielding is willing to engage upon and something upon which there can be a true sense of embrace.  Yet, Aziz is not that introspectively drawn.  Aziz won't even advocate his own Islamic faith as part of this process, rather dismissing the entire element.  For Aziz, exploration and liberation are on a political sphere.  For Fielding, exploration and liberation are probably more on a spiritual one, constructing another barrier that prevents their friendship from fruition.