Welshman Ncube, chief negotiator of the smaller Movement for Democratic Change (MDC) faction, said southern African leaders, Mugabe and his group saw this as a reasonable power-sharing arrangement.
It was the clearest indication to date of the proposal being discussed. A media blackout has been imposed on talks.
South Africa’s The Star newspaper quoted Ncube on Thursday as saying in an interview that both the president and prime minister would share power under the proposal.
"This is where an executive president has executive power and an executive prime minister has executive power and they have to share this power and they have to make decisions between themselves," said Ncube, the faction’s secretary-general.
"Some of them by consensus, so that no one feels that they have been booted out of holding executive authority."
The question of who gets executive powers is the main sticking point in the negotiations aimed at ending Zimbabwe’s political crisis, which has deepened since Mugabe’s re-election unopposed in June in a poll boycotted by Tsvangirai.
Ncube said the Southern African Development Community (SADC) regional grouping has been trying to convince all parties to accept the proposal.
When asked if such a proposal was on the table, Tsvangirai spokesman George Sibotshiwe said he could not discuss the details of the negotiations.
Power-sharing talks began last month after Mugabe was re-elected in a vote condemned around the world and boycotted by Tsvangirai because of attacks on his supporters.
Ncube said all parties had initially agreed to documents which gave the president the powers to chair the cabinet and made the prime minister his deputy.
The breakaway MDC faction would seek to take the position of Speaker of the lower House of Assembly when Zimbabwe’s parliament convenes on Monday, a party spokesman told state media on Thursday.
The Speakership is a senior position in Zimbabwe’s political hierarchy.
"The position (to seek the Speaker’s post) is premised on the view that it is our party that wields the balance of power," the faction’s spokesman Edwin Mushoriwa said.
Neither of the two major parties holds a parliamentary majority — Tsvangirai’s main MDC holds 100 seats in the lower house of parliament, against the ruling ZANU-PF’s 99.
The breakaway MDC has 10 seats and whoever it sides with gets an effective majority in the legislative chamber. Reuters