They were once allies, standing shoulder to shoulder against the August 1991 coup attempt. Now they are at each other's throats. And for the second time in two months Mr Zorkin has been called in to break up the fight.
Today he brings them together in the Kremlin in an attempt to end their latest bout of name-calling and political pugilism.
As head of the Russia's highest court Mr Zorkin tries to stay above the fray. His involvement as a peace-keeper, he promised, 'does not mean a third muzhik is appearing to join the fight . . . we should act as an arbiter'.
The brawl is more than a personal feud, though this has a lot to do with it. It involves the great unresolved issue of post-Communist politics in Russia: who has ultimate authority?
Mr Khasbulatov, who chairs both the Congress of People's Deputies and the smaller standing legislature, the Supreme Soviet, says parliament should have the final word. Mr Yeltsin insists he is boss. There are policy issues at stake, too, though Mr Khasbulatov himself has done little more than suggest that Russia should follow the 'Scandinavian model' rather than the 'shock-therapy' favoured by Mr Yeltsin's American-influenced economic advisers.
Parliament as a whole, though, is deeply conservative and keeps doing its best to dismember reform. Last month it voted a 90 per cent increase in pensions, upsetting the government's budget plans for the year. It has tried, without much success, to slow privatisation and keeps sniping at Russia's liberal Foreign Minister, Andrei Kozyrev. The central issue, though, is power.
It was supposed to have been decided with a referendum, scheduled for 11 April, on the shape of a new constitution to replace one inherited from the Soviet era. The current constitution has been revised dozens of times and is a ramshackle document that Mr Yeltsin insists must be replaced if Russia is ever to break a political deadlock and get on with reform.
But the referendum, opposed by Mr Khasbulatov, who feared a reduction in parliamentary power, and regional leaders who warned of an 'uncontrollable struggle for power', now seems to be off. Mr Yeltsin, on the urging of Mr Zorkin's Constitutional Court, has called for a 'year-long moratorium on all political fist-fighting'. The referendum, he says, can wait. Instead, there should be new parliamentary elections in 1994 and a new presidential poll in 1995 - both a year ahead of schedule.
What happens now depends on today's talks between Mr Yeltsin and Mr Khasbulatov. But as with a previous round of negotiations between the two men in December, also refereed by Mr Zorkin, Mr Yeltsin is on the defensive.
Though appointed rather than elected and far less popular than Mr Yeltsin (he is hampered by his non-Russian origins in the Chechen republic), Mr Khasbulatov, an obscure professor until two years ago, has now gone two rounds with the President and seems to have won both.
The first was in December, during the last session of the Congress of People's Deputies, a body elected under Communism and stacked with time-servers from the Soviet era. After savaging the government's reformist policies in floor debate, deputies forced Mr Yeltsin to dump his economic guru, Yegor Gaidar, as prime minister. Before dropping him, though, Mr Yeltsin, after his last power-sharing talks with Mr Khasbulatov, claimed one important concession from his parliamentary foes: the promise of a constitutional referendum.
Such a vote, insisted Mr Yeltsin and his allies, would fix once and for all the balance of power between the executive and legislative branches. 'There is the President on one side and parliament on the other,' said the Deputy Prime Minister, Vladimir Shumeiko, last month. 'We need a referendum to decide which half the people want, which they trust.'
Suddenly, though, the referendum is in doubt. Round two to Mr Khasbulatov. He has opposed it from the start, depite having agreed to it in December, and has sharply stepped up criticism of Mr Yeltsin.