Their rivalry has fuelled appalling bloodshed in South Sudan, yet now for a third time Salva Kiir and Riek Machar will try to share power in the latest bid to bring peace to the world's youngest nation.
A deal signed on Sunday between the rivals and other opposition parties under the mediation of Khartoum was welcomed with celebrations from the capital Juba to camps housing those displaced during five years of war.
Observers warn of a long road littered with obstacles between that signature and a final peace deal with rebel leader Machar's return from exile to Juba to serve as first vice president.
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Getting Machar and President Kiir to work together is a tricky business, for the combination has always resulted in chaos and conflict in the past.
"It is going to be a real struggle because in meetings with President Kiir, he has made it pretty clear he didn't want to work with Machar," a diplomat based in Juba told AFP on condition of anonymity.
"He has really had his arm twisted into accepting the deal again."
Lack of trust
Both Kiir and Machar are former rebel leaders who rose to power during Sudan's 1983-2005 civil war between north and south - a conflict in which the men also fought each other - before South Sudan gained independence in 2011.
The two leaders come from the south's two main ethnic groups - Kiir from the Dinka and Machar from the Nuer.
Their cohabitation in South Sudan's first independent government came to a swift end in 2013 when Kiir accused Machar of plotting a coup against him, kickstarting a war driven by ethnic conflict between their two communities.
A peace deal pushed the two men into the same government again in 2016, but just months after Machar's return fighting erupted in Juba and he was forced to flee on foot with his supporters to the neighbouring Democratic Republic of Congo.
"I don't know how, with that level of violence, you can have trust in each other's actions and have a working government," said Ahmed Soliman, a Horn of Africa researcher with Chatham House.
The fighting that followed was worse than before, and infinitely more complex, with new rebel groups and grievances fuelling bloodshed, famine and displacement.
The latest peace push has been led by Uganda and Sudan, which is suffering an economic crisis and is keen to see oil start flowing again from South Sudan, whose economy has completely collapsed. Like the north, it desperately needs the income.
The sharing of this oil wealth is one of the contentious issues in the peace deal, which provides for an escrow account in which all parties will have a stake while the government will hold the most control.
"Will the government use this money for the purpose it is intended, or will it go back to its old ways where a few people dig their hands into the coffers and run away with the money," said Brian Adeba of The Enough Project advocacy group that closely follows the South Sudan conflict
'A portion of the pie'
The evolution of the conflict has brought many more rebel groups to the table, resulting in a unity government with five vice presidents, a 35-minister transitional government and a 550-member parliament.
"This is very much a government of accommodation, let's be honest, this is about trying to give people status, a portion of the pie. That I think doesn't bode well for a transitional government and the nature of it," said Soliman.
Meanwhile it is unclear whether Machar's rank and file will accept the deal he has signed, and there are still rebel groups - such as that of former army chief Paul Malong - which are outside the deal.
Kiir has vowed the deal "will not collapse", but has already complained about the bloated government and questioned how it will be funded.
"They need security, they need vehicles, they need houses... five vice presidents, this is a very big responsibility to manage," he said this week.
Drop in fighting
The warring parties now have to hammer out a final peace deal, and will then have three months to form a transitional government which will hold power for three years.
While many questions remain over the cantonment of soldiers, security for Machar's return and details of governance and reconciliation, observers say that despite violations, a July 27 ceasefire has led to a decrease in violence.
"We have noticed a big drop off in fighting. There are still skirmishes but nothing like the magnitude that we had before," said the diplomat in Juba.
"As long as fighting stops it means ordinary people can get on with their lives and we can start resolving this massive humanitarian crisis."