"They seem to be getting along fairly well," Motlanthe said of the two Zimbabwe leaders, who are preparing to formally share power under a regional-brokered deal after years of feuding.
"We are optimistic that they can at least manage a transition period until they are ready to hold fresh elections," Motlanthe said.
Tsvangirai is set to be sworn in as prime minister on Wednesday under a power-sharing deal South Africa mediated between the opposition leader and Mugabe in September.
The pact aims to end almost a year of intense political turmoil following disputed March 2008 elections, in which Tsvangirai’s party seized a parliamentary majority in the first round.
"Whether they like it or not, or whether they like each other or not, they are bound to work together if anything is to be passed by that assembly and if the country itself is to pull itself out of poverty and disintegration of its infrastructure," said Motlanthe.
Political analysts have said they doubt a union government will work, citing a deeply-rooted lack of confidence between the two men.
Mugabe has frequently referred to his adversary as a Western "lackey" or "puppet", while Tsvangirai has accused Mugabe, in power since 1980, of human rights violations.