THE DIPLOMACY of the Mideast has turned not only a hopeful corner but an ironic one. No sooner had President Anwar Sadat of Egypt made peace seem possible for the first time in 30 years than the parties and outside powers began insisting that peace between Egypt and Israel alone would be a mistake. Cairo and Jerusalem are now outdoing each other in professions of desire for a comprehensive settlement - though no other Arab country has yet publicly joined the process and Israel has yet to complete the policy review that alone would make its professions credible. The United States demonstratively agrees that the conference opening in Cairo on Wednesday should try not to make a separate deal but to move toward an overall accord.

Two things need to be said. The first is that, strategically speaking, it would be irresponsible to shoot for a separate peace. The very effort would strengthen those Arab elements, inside as well as outside Egypt, determined to bring down President Sadat. He would risk alienation of the loyalities and subsides of the Saudis, whose cooperation in oil and money happens to be vital to the United States. The Palestinian problem would fester. The risk would increase that Arab "rejectionists" would use, and be used by, a Soviet Union angered by exclusion from a leading Mideast role.

But, tactically speaking, it would be foolish and self-denying not to leave open the possibility of a separate peace. It is precisely the prospect of being frozen out that is most likely to draw into the peace process the parties now outside it. Syria's president , in his consultations with the Saudis and other Arabs, is almost certaintly canvassing the terms on which he might come in from the cold. If anything will induce the PLO to rethink its rejectionism, it is the spectacle of West Bank-Gaza Palestinians responding to the hand offered them by Israel and Egypt. The Russians must figure that an Israeli-Egyptian deal, broadened as it might be to include a Saudi-backed Jordanian West Bank combination, would leave Moscow way out in left field with the losers of Syria and the PLO. Mr. Sadat has enabled the United States to solicit moderation in Moscow and Damascus knowing (and knowing that they know) that their cooperation is useful - especially to them - but not essential to the project at hand.

To pursue a comprehensive peace even while leaving open the possibility of a limited deal takes some diplomatic doing. This appears to be the common policy of Egypt, Israel and the United States. It provides a basis for hope that the Cairo conference, which only Egypt and Israel among the parties whill attend but which will be open-ended in time, agenda and participation, will keepup the momentum that Mr. Sadat began.