Only two weeks after their armies were locked in bitter border combat, North and South Yemen today announced a plan to unify their countries.

The unity plan, which is subject to further negotiation on timing and implementation, is yet another of the startling turnabouts that mark affairs in this strategic region of the Arab world.

An earlier unity agreement between pro-Western North Yemen and the Marxist and Soviet-backed South, reached after border clashes in 1972, dissolved as the enmity that has marked relations between the two resurfaced.

Arab world specialists remained extremely skeptical that the latest proposal would stand a better chance of being carried out than previous endeavors, given the ideological diversity and the legacy of bitterness between the two.

Other unification efforts in the Arab world, involving at various times Egypt, Libya, Syria and Iraq, also have failed to come to fruition or have fallen apart.

The latest announced merger plan raises a particular problem for the United States, which responded to the recent hostilities by announcing that it was sending $400 million in Saudifinanced armaments to North Yemen.

Under the proposal announced today, defense and foreign affairs issues would fall under the unified government, raising the theoretical possibility that the U.S. arms sent to aid the North Yemenis would also be avialable to their former foes in the South.

Under the agreement, the unified country will be called the People's Republic of Yemen, with Sanaa, now the North Yemeni capital, as capital of the new country, the Kuwaiti news agency reported.

Specialists recalled, however, that in addition to the latest border warfare, in which the Soviet and Cubanbacked South Yemenis thrashed the North's forces, there are memories of last year's assassination of North Yemen's president by a briefcase bomb apparently sent by South Yemen.

The specialists noted today's announcement at a minimum is a victory for the Arab League, which intervened to halt the fighting and apparently has won at least a respite from renewal of th conflict.

Under the agreement reached at a summit meeting here, a constitutional committee will have six months to draft a charter for the unifed state. It would then be submitted to a referendum in both countries.

North Yemen's Ali Abdullah Salem and South Yemen's Abdul-Fattah Ismail signed the agreement at Al Salam Palace, the Kuwaiti announcement said.

The agreement calls for a Sovereignty Council and an interim government to administer the territory until the elections, but no details were announced.

The agreement, details of which were vague, apparently envisions afederated country with internal matters remaining under territorial governments corresponding to the present two countries.