I need assistance in finding two significant figures, dignitaries, whatever it may be that started on good terms, as allies, but began to have opposing thoughts/views that led to their eventual rivalry?
You mean apart from Caesar and Pompey?
Just about every Republican figure had severe criticisms of every other Republican figure after the (Edit, thanks /u/gh333) Spanish Civil War (which the Republicans lost). Their arguments were ongoing and quite bitter, despite them being allies during the war (ranging from allies of convenience to true friendships).
The fall of the Ming dynasty involved several of these situations, notably with Li Zicheng (imperial general turned warlord turned usurper), Wu Sangui (gate guard, allies with the barbarians outside the gate to get rid of the warlord within, ends up betraying the barbarians later) and the various leaders of the Manchu tribes and early Qing dynasty who befriended, betrayed and were betrayed by various Ming leaders as they cemented their rule over China.
The relationship between Wu Sangui and the Manchu/Qing leader Dorgon fits your prompt rather well as they began as rivals each seeking to thwart the other, became allies of necessity when the Ming crumbled from within, and the two sides later became enemies once more when Wu declared himself emperor and was crushed by one of Dorgons successors, the Kanxi emperor of the Qing.