At a recent breakfast with reporters, Zbigniew Brzezinski, Jimmy Carter's national security adviser, was asked about the Haig-Weinberger duet in foreign policy.

Brzezinski: We are seeing, perhaps, the birth pangs of a policy. The question is whether it will be stillborn.

Voice: It's twins!

Brzezinski: Not Siamese, unfortunately. (Laughter)

The apparent discord over U.S. arms sale policy in the Middle East, which resulted last week in a major flap with Israel, seems funny in a warm and secure Washington hotel room. But it clearly doesn't amuse Menachem Begin, for whose country it could have dire consequences.

In 1976 the Ford administration committed the United States not to sell mobile anti-aircraft missiles to Arab states. Jordan's anti-aircraft batteries stand now on fixed sites known to Israeli intelligence, making the military balance in that respect predictable and stable.

Begin, accordingly, was as unamused as Queen Victoria to read that Defense Secretary Caspar Weinberger was talking with Jordan's King Hussein about selling mobile anti-aircraft missiles.

Hussein's grandfather, King Abdullah, was assassinated in 1948, probably for being gracious about the founding of Israel. Abdullah's grandson is a sour little monarch, unhappy with everyone's policies, whose regime the Israelis saved from Syrian assault 12 years ago. But no good deed goes unpunished, as the saying runs, and Hussein, by threatening to buy his arms from Russia, is putting the squeeze on the United States to sell him mobile anti-aircraft missiles--a sale that could destabilize his relationship with Israel.

Hussein's shopping list, and Weinberger's willingness to discuss it, ignited Begin's wrath and resulted in a nearly unanimous resolution by the Israeli parliament. The resolution doesn't tell Ronald Reagan how to balance the U.S. commitment to Israel's security against the clamor of Arab states for high-tech weaponry which they are likely to use against one another--or Israel. Unlike the underlying problem, the resolution is borrowed trouble. It was attributable to obscure musings aboard Weinberger's plane about a "redirection" of U.S. Middle Eastern policy.

The Israelis are also aware Weinberger successfully advocated Reagan's decision to sell AWACS aircraft to Saudi Arabia. This is another decision that threatens Israel's air supremacy, its lifeline. It is Weinberger's policy to pacify the "moderate" Arab regimes with the sale of advanced weaponry, although all of them (with the exception of Egypt) remain immoderately hostile to Israel's existence. Hence Caspar Weinberger would be a questionable emissary, even if the arms-sale policy were well considered.

Menachem Begin has other problems as well. He is under harsh pressure at home to unleash the Israeli army against PLO concentrations in southern Lebanon, now heavily resupplied by the Soviet Union in violation of understandings negotiated last summer by U.S. Ambassador Philip Habib. Were it not for Begin's scruples about notifying the United States beforehand, the Israeli strike would probably have occurred a month ago, and it remains a lively possibility.

Imagine, then, the effect of Weinberger's unguarded talk on an Israeli prime minister who is holding his generals on a frayed leash and contemplating the painful April 25 deadline for restoring the last segment of the occupied Sinai to Egypt.

Reagan presumably tolerates Weinberger's frequent personal improvisations in policy (not only about arms sales to the Arabs but his recent dissent over the Polish loan issue) because he's an old friend and confidant. But Weinberger's roving commission is a costly indulgence.

When the latest episode demanded some hasty firefighting by the president, we were told that it was the result of a misunderstanding fostered by "press reports" and "exaggerated commentary." Haig, in a smirking television interview, called the problem "a not-too-unusual firestorm in Washington press circles," perhaps the failure of a reporter to hear correctly "a caveated statement."

If there was some misinterpretation, which is possible, it is hardly the root of the problem. What actually needs to be "caveated," in Haig-speak, is Weinberger's preoccupation with the military side of foreign policy and his unwillingness to subdue personal differences with the secretary of state, even when Haig's view is official U.S. policy.

If the confusion is prolonged, the Reagan administration will find itself with a foreign policy problem that can't be handled by soothing letters to foreign leaders or blamed on bad reporting.