It is one of Mugabe's weaknesses that he has always preferred to keep closest to him lesser individuals who he can control. The reason he has always played Mujuru and Mnangagwa off against each other is that each presents a real threat threat to his position. Neither Chiwenga or Chihuri present any such threat - both were saved from ignominy by Mugabe and their future would be nothing without him. Both men were considerably junior to the now much less powerful Zimondi in 1979. Mugabe hand-picked them not for their strengths but their weaknesses.
Chiwenga's major weakness of course was his failure to make the grade at the end of his training course in Gweru in the early eighties. This was such a personal setback that he was driven to kill himself - the only problem was that he failed in that as well. The bullet intended for his heart passed straight through his chest and out his back without hitting the intended target.
Often people consider failed suicide attempts to be really cries for help. There is no suggestion that this was anything other than a genuine attempt but it served as the most effective cry for help Chiwenga could possibly have made. The then PM, Robert Mugabe, took pity on him and got the instructors involved to reverse their decision. He then got Chiwenga promoted to Brigadier in the first batch of post-independence commanders of the army's four brigades. Rarely, if ever, has anybody's rapid promotion through the ranks been so genuinely accidental.
Chihuri's rise owes as little to competence as Chiwenga's and was if anything even more unlikely. In the late seventies, Chihuri was under arrest - but not by the white regime in the then Rhodesia for his activity against the Smith regime, but in Mozambique for plotting against Robert Mugabe! Indeed he was only released at the insistence of the then Governor, Lord Soames.
Although most former guerillas were integrated into the national army, Chihuri had to settle for the police which he joined as a lowly patrol officer. He progressed rapidly through the ranks despite (or because of?) a growing reputation for corruption - indeed despite being the officer in charge of Nyanyadzi police station in Manicaland when the police funds there were misappropriated, he was transferred from there to Mutare on promotion to superintendent while the case was being investigated - by Mutare CID. His subsequent rise to Commissioner was accompanied by further scandal, particularly during his period as Deputy Commissioner, and it is thought that the hold Mugabe has on him as a result ensures his long-term and unswerving loyalty.
The current rumours about Chiwenga's aspirations for the Presidency in the event of Mugabe's death may owe something to the recent downturn in the political fortunes of Emmerson Mnangagwa. Chiwenga is not someone with a political power base of his own but he would feel very vulnerable if his current political patron lost power - for whatever reason. Because of their close co-operation on the JOC, Mnangagwa would have been best placed to provide political top cover for Chiwenga in the future. But Mnangagwa's dramatic loss of influence in recent weeks might conceivably have caused him to consider other options. Whatever he might be considering however, he could never be a serious contender for the top job and if he tried to use his current position to secure it, neither the party or the people would tolerate it.