Opposition leader Morgan Tsvangirai is set to be sworn in on Wednesday as prime minister, with long-time ruler Robert Mugabe remaining as president.
South African President Kgalema Motlanthe said that so far the two rivals "seem to be getting along fairly well".
"We are optimistic that they can at least manage a transition period until they are ready to hold fresh elections," he told South African media.
But analysts said the union was a shotgun wedding that Tsvangirai only agreed to after coming under enormous pressure from regional leaders frustrated at the long months of stalemate.
"The levels of mistrust between the two main principals will remain irreversibly high, leading to threats of pulling out as well as manoeuvres to get fresh elections as soon as is politically possible," said Takura Zhangazha, director of the Zimbabwe chapter of the Media Institute of Southern Africa.
While African leaders are throwing their weight behind the unity government, Western powers are reticent, with Washington and London saying they want to see improvements in the running of Zimbabwe before they will lift a travel ban and asset freeze on Mugabe.
"It will never achieve total international support in its current form and therefore will be unable to address the political and humanitarian crisis effectively," Zhangazha said.
The deal was sealed on Thursday after nearly seven years of talks and a series of disputed elections.
Tensions came to a head last March, when Tsvangirai won a first-round presidential vote and his Movement for Democratic Change (MDC) captured a majority in parliament.
That unleashed a new wave of the political violence that has shaken Zimbabwe since Mugabe’s first defeat at the polls in a referendum in 2000. Most of the victims have been opposition supporters, leading Tsvangirai to pull out of the presidential run-off.
Mugabe declared a one-sided victory denounced by Western powers, sparking months of frenzied lobbying by South Africa to win the power-sharing deal now finally set to take effect.
MDC lame ducks?
The Harare-based independent political analyst Martin Tarusenga said that Mugabe has tried to dictate the terms of negotiations throughout the years of talks, and that the unity government would not last without international support.
"Judging by statements made from the West so far, the international community is not seeing it as an inclusive government," he told AFP.
"Tsvangirai and his MDC are now looking like lame ducks so the international community will not support the inclusive government," Tarusenga said.
"There is just no trust. Tsvangirai is buckling from SADC pressure and the pressure from weaker MDC negotiators, those people in the MDC who believe they have fought a good fight."
The Southern African Development Community (SADC) has pushed for the deal as the best way to end Zimbabwe’s stunning economic collapse, with hyperinflation soaring to astronomical heights.
Only six percent of the workforce actually has a job, more than half the population needs emergency food aid, and a cholera epidemic is raging unchecked, claiming more than 3 300 lives.
Political analyst Bornwell Chakaodza said he feared that if the unity government failed, Zimbabwe’s crisis could still get even worse.
‘A violent uprising’
"Failure of the inclusive government will be an indication that there can no longer be a negotiated and peaceful settlement to the Zimbabwean political conflict," Chakaodza wrote in his weekly column in the Financial Gazette on Friday.
"The alternative would be a violent uprising whose consequences we dare not imagine," he said.
Zhangazha said he feared that rivals within the government would spend more time feuding within the government than working to solve the nation’s problems.
The main issue "will really be about the politics, and about outmanoeuvring each other, even at the expense of the masses", he said. – AFP