wo weeks after publicly splitting over a Security Council condemnation of South Africa, the five members of the Western "contact group" negotiating an independence plan for Namibia today managed to present a united front on the same issue in the U.N. General Assembly.

The five--Britain, France, West Germany, Canada and the United States--joined 20 other primarily Western nations in abstaining on the 117-0 vote of a special General Assembly session to condemn South Africa's invasion of Angola last month in pursuit of Namibian guerrillas.

But the United States--which last month was alone in its veto of a similar Security Council condemnation--and the other four contact group members, made it clear their unanimity was based principally on the fact that the General Assembly vote "didn't mean anything." General Assembly resolutions, as opposed to those in the Security Council, are not binding.

Rather, the United States on one side, and the other four members on the other, appeared to want to smooth over their growing differences on Namibia and on the extent to which South Africa, which controls the territory, should be pressured to agree to an independent plan, before their foreign policy officials meet here Sept. 24.

In private conversations today, European delegates indicated their governments were rapidly losing patience with the Reagan administration's policy of "constructive engagement" with South Africa.

In explaining the U.S. abstention, Ambassador Jeane Kirkpatrick said it was "on purely procedural grounds, notwithstanding our well-known position on the substance."

Calling South Africa a country that is a "democracy for whites and a dictatorship for blacks," Kirkpatrick said that U.S. leverage there could best be used by "maintaining normal relations and making clear" U.S. disapproval of Pretoria's policies.

According to one European delegate to the contact group, "The five stuck together--just--in agreeing that we would jointly abstain."

In its July 31 Security Council veto the United States argued that to condemn South Africa alone for its raids was one-sided and ignored Angolan-based aggression against South Africa-controlled Namibia (South-West Africa) and the presence of Cuban troops and Soviet military personnel in Angola.

Three of the four group members--France in a Security Council debate and West Germany and Canada in statements--condemned South Africa without qualification, while Britain abstained.

Shortly after the Security Council veto, Secretary of State Alexander M. Haig Jr. reported that the United States had made more progress in moving South Africa toward a Namibian settlement than had been made in the past three years.

Although neither Pretoria nor Washington has commented publicly, the United States is said to have persuaded South Africa to accept U.N. Resolution 435 as a basis for settlement, and to accept the presence of a U.N. peacekeeping force to supervise the accord.

Resolution 435, unanimously approved here in 1978, calls for a cease-fire in Namibia, elections, a constituent assembly and a national constitution.

"Perhaps we've all been at it too long," said one European delegate today who has been active in negotiations since 1978. "The points that to Haig may seem like advances don't seem like all that much."

Besides adhering to Resolution 435, one European delegate said, South Africa must agree to specific dates for effecting the agreement and demilitarizing Namibia.

A third element is agreement to the "constitutional elements" of the settlement. Here the issue is whether, as a European said, a constitution with guarantees for minorities "would be handed" to Namibia before elections that Soviet-backed SWAPO would be expected to win. The Reagan administration has indicated it favors this route.