They said they hoped Bennett and other detainees would soon be released on bail.
Motlanthe and Tsvangirai met at Tuynhuis in Cape Town to discuss the situation in Zimbabwe and ways to get the country back on track.
The Movement for Democratic Change reiterated accusations that Bennett’s detention on terrorism and weapons charges was politically motivated.
Dead body in cell
"The conditions in the prison are so deplorable that one person in Roy Bennett’s cell died yesterday and the body is still to be removed. Prisoners are literally starving to death," it said in a statement.
In addition to acute food shortages, Zimbabwe is in the midst of a cholera epidemic blamed on its crumbling health care and infrastructure. The World Health Organisation said Friday that the number of cases has soared above 80,000, including 3 759 deaths.
Despite the problems, President Robert Mugabe is planning a lavish party next week to mark his 85th birthday, which is on Saturday.
Save The Children sent him a birthday message.
"As Mugabe throws parties in Zimbabwe for his 85th birthday, one in 10 children in his country are destined to die before their fifth birthday. Most of their mothers won’t even live to half the president’s age," it said.
"There’s nothing for children to celebrate in Zimbabwe. As thousands of pounds are spent on birthday food and drink, millions of children struggle to survive on basic food aid rations, often with no way of getting clean water.
Tsvangirai said on Friday that his country could need as much as $5bn in aid.
Motlanthe said southern African finance ministers and the head of the African Development Bank would meet next week to evaluate the needs and said South Africa was prepared to take the lead in any financial rescue package.
"We are acting as good neighbours," Motlanthe said.