The honeymoon is over for Socialist President Francois Mitterrand and conservative Prime Minister Jacques Chirac, joined in a political shotgun marriage for the past 10 weeks.

Forced into a unique power-sharing arrangement known as "cohabitation" after a narrow right-wing victory in parliamentary elections, the two leaders have begun to prepare the ground for their eventual divorce. Like couples in the early stages of an estrangement, each is eager to be able to prove that it was all the other's fault.

"It's cohabi-tension," exclaimed the French satirical weekly Le Canard Enchaine in a banner front-page headline last week. Other newspapers published cartoons showing the two political leaders lying in ambush for each other or struggling for control of the steering wheel of a ramshackle car.

The new, more turbulent phase of "cohabitation" follows a series of well-publicized disagreements between Chirac and Mitterrand, whose seven-year presidential term does not expire until 1988. It contrasts with the relatively tranquil period immediately after the March 16 election when both men had an interest in demonstrating that it was possible to accomplish a smooth political transition after five years of left-wing rule.

The issues in dispute have ranged from the Reagan administration's Strategic Defense Initiative, which Chirac supports but Mitterrand does not, to proposed changes in the electoral system. Also at stake is control over the government bureaucracy, with the prime minister challenging several high-level appointments made by the president before the election.

Further complicating Chirac's life is the fact that many of his supporters want him to move much faster in dismantling the Socialist heritage. A recent editorial in the right-wing Paris daily Le Figaro warned that France's neoconservative revolution was in danger of getting bogged down in parliamentary procedure and government inaction.

Responding to the pressure, Chirac twice moved to cut short the debate in the National Assembly on electoral reform and a new budget, overriding opposition censure motions. He also allowed the minister for overseas territories, Bernard Pons, to argue with the president during a weekly Cabinet meeting, an act of lese majeste unprecedented in the 28-year history of the French Fifth Republic.

Right-wing politicians suspect that Mitterrand's strategy is to win points with public opinion by posing as an impartial constitutional referee while distancing himself from unpopular measures. Executive power has largely shifted since the election from the Elysee presidential palace to the Matignon, where the prime minister has his offices.

The latest public opinion polls show that "cohabitation" still has the support of a significant majority of voters, but the government's popularity is beginning to slip from its postelection peak. Mitterrand, on the other hand, is more popular than at any time since 1982, with a 50 percent approval rating.

Chirac's spokesman, Denis Baudouin, last week compared "cohabitation" to a flight over uncharted territory. Chirac's problem is that, although he has control of the plane, Mitterrand has the power to decide how long the flight is going to last. As head of state, Mitterrand has the constitutional means to dissolve the National Assembly, call a referendum or provoke new presidential elections by resigning. As he reminded journalists recently, "the president is the master of the political timetable."

The power to put an end to "cohabitation" is a double-edged sword, however. French voters are unlikely to reward someone they regard as responsible for precipitating a political crisis. As one of Chirac's supporters, Jean-Claude Gaudin, remarked recently: "Cohabitation is the opposite of a western: the one who draws his gun first loses."

Another paradoxical feature of the situation is that it has allowed Mitterrand, a bitter opponent of Gen. Charles de Gaulle, to pose as the defender of France's Gaullist institutions. The task of dismantling de Gaulle's most important legacy, a strong presidency, has fallen to Chirac, the head of the neo-Gaullist Rally for the Republic party.

Mitterrand has also assumed the Gaullist mantle by his opposition to the Strategic Defense Initiative, popularly known as "Star Wars," which he sees as a potential threat to France's independent nuclear deterrent. In a speech last week, he reminded officer cadets that the principle of mutually assured destruction remains the centerpiece of French defense strategy.

The speech was widely interpreted as a reply to remarks by Chirac the previous week, describing the Strategic Defense Initiative as "inevitable, irreversible and justified."

As part of the continuing struggle over the ground rules for cohabitation, Chirac has sought to leave his stamp on French foreign policy, traditionally regarded as a presidential prerogative. Last month, he flew to Tunisia to reassure ailing President Habib Bourguiba of French support in the event of a conflict with neighboring Libya.

The current obsession with "cohabitation" is reflected by the fact that interest centered not on how Chirac would get on with Bourguiba, but how he would hit it off with French Ambassador Eric Rouleau, a Mitterrand nominee known for his Socialist sympathies.

In fact, Chirac refused to meet his ambassador. Rouleau was recalled to Paris for hastily arranged "consultations," one of the first victims of "cohabitation, phase two."