"Where do you separate reality from theater in diplomatic negotiations?" a weary British diplomat asked last week at the end of another frustrating day of talks at the Rhodesian settlement conference.

For white former prime minister Ian Smith, who has made a career of hard-nosed negotiations with the British since illegayly declaring Rhodesian independence in 1965, reality struck home yesterday. He was isolated as the only opponent of a British-proposed independence constitution in the Zimbabwe-Rhodesian delegation of black Prime Minister Abel Muzorewa.

For the Patriotic Front fighting a guerrilla war against the Muzorewa government, the first two weeks of the British-sponsored conference have been mainly theater with the Front stealing the show.

Muzorewa's agreement yesterday on the constitution, however, means that Front leaders Robert Mugabe and Joshua Nkomo now face the reality that it is their turn to make concessions. The odds are fairly strong that after a bit more time to test the will of British Foreign Secretary Lord Carrington in the same manner Smith did, the Front will come around to a tentative agreement on the British proposals.

The British diplomat predicted earlier in the week that it was likely that there would be some "kicking and screaming" from both sides to gain the maximum credit for concessions but said the conference was unlikely to break down over the issue of the constitution.

Although such an agreement on the constitution involving the three sides would be far from the long-sought Rhodesian settlement, it would be the first time the guerrillas and the government have reached accord on any document and could give momentum toward an eventual settlement.

As always with the intractable Rhodesian situation there are many complexities and the chances of a settlement are well under 50-50, but perhaps those odds are not bad in comparison with the escalating guerrilla war in which upwards of 100 persons a day are being killed.

Ironically, one of the complexities may end up being the role of the U.S. Congress since there is a strong possibility that it may vote to lift sanctions against Zimbabwe-Rhodesia early next month as part of a defense appropriations bill. This would increase pressure on British Prime Minister Margaret Thatcher at next month's Conservative Party conference to follow suit.

Such moves could destroy the delicate diplomacy Carrington has set in motion to bring about a negotiated settlement. It would mean the Muzorewa government would finally have realistic hope of gaining international recognition and an end to sanctions, without making a deal with the Front.

Satisfaction of those two demands is why the Muzorewa government came to the 13-day-old London conference, but it will not end the war that the British government has repeatedly said is the overall purpose of the talks.

Having agreed to the British-proposed constitution that removes key elements of government control by Zimbabwe-Rhodesia's 230,000 whites, Muzorewa can repeat his claim of "What more must we do to satisfy the world?" That is the very thing his conservative supporters in the United States and Britain ask.

The pressure is thus on the Patriotic Front to agree on the constitution so the conference can move on and begin to answer the "what more" question: How to implement the constitution through a cease-fire, disposition of the warring forces and establishment of a transitional government to organize new elections.

Muzorewa would prefer to take the new constitution and have his presently unrecognized government implement it without any elections. Britain, the legal colonial power, is committed to seeking an overall settlement. Without it, the Patriotic Front has vowed to continue fighting.

The Front is far more interested in the transitional arrangements that will involve jockeying for eventual power than in a constitution that can be changed after gaining power. Thus, Britain is gambling that faced with this pressure the Front will agree to guarantee some seats for whites in Parliament -- the key constitutional issue -- to move on to talks on the transition.

The danger for Muzorewa's limited black-majority government is that in moving to the second phase of the conference it is involved in negotiating away its very existence.

Britain, in effect, has the middle ground, and is seeking to force both sides to come toward it in the interests of a settlement.

Carrington, who is proving to both sides that he is a tough negotiator, can tell the Muzorewa government that without an overall settlement the goal of recognition and an end to sanctions cannot be achieved.