The Rhodesian security forces and the Patriotic Front guerrilla armies have reached an understanding to begin limited joint training exercises, a major achievement that could greatly ease fears of a renewal of warfare after next week's election of a black-majority government.

In another significant step toward binding up the wounds of seven years of war, informed sources said today that two senior Rhodesian military officers are visiting for the first time the 14 assembly camps where the guerrillas are gathered. The move is part of a British effort to acquaint the two sides with each other to work toward building a joint army.

Maj. Gen. John Ackland, commander of the Commonwealth ceasefire monitoring force, said today that the understanding on joint training was "the biggest achievement since the cease-fire," which is supervised by Britain, took effect Dec. 28.

Speaking of the uncertain military situation following the elections, which run from Wednesday through Friday, Ackland said, "You can have nine political parties at independence" as Rhodesia does, "but you cannot have three armies."

At present the two armies of the Patriotic Front have a total of 22,000 troops in the assembly bases while the British colonial governor, Lord Soames, controls the 30,000-man Rhodesian security forces, backed up by a call-up of about twice that number of reservists.

There is considerable concern that once the 1,200-man Commonwealth cease-fire monitoring force withdraws, the well-armed guerrillas could leave the camps or the Rhodesian forces might attack them at the bases. Either possibility could lead to a bloody resumption of the war.

Over the weekend Joshua Nkomo and Robert Mugabe, the two guerrilla leaders, agreed to a request by Soames to order their troops to remain at the assembly points after the election but the announcements have yet to be made.

The joint training to begin this week at a base near the southwestern city of Bulawayo is seen as the first important step toward building a new army, composed of elements from the security forces and the two guerrilla organizations, for the independent nation of Zimbabwe that is to emerge next month.

British officials declined to give details of the planned joint training but said an announcement by the three sides was expected soon.

Negotiations, under way for several weeks, were reportedly given impetus last week by a meeting between Gen. Peter Walls, commander of the Rhodesian military, and Nkomo, leader of the Zimbabwe People's Revolutionary Army, the smaller wing of the Patriotic Front military alliance.

It was learned that the program will initially involve several hundred of Nkomo's troops training under the supervision of the monitoring force and with the assistance of a smaller number of Rhodesian officers and senior enlisted men.

The other military wing of the Front, Mugabe's Zimbabwe African National Libertation Army, is known also to be enthusiastic in principal about the plan but has not yet given final approval although it is expected.

A source close to the military situation said all sides agreed that the eventual melding of the armies must come about if an independent Zimbabwe is to have any chance to put the war behind.

Even though the program was only a limited pilot scheme, the source said it was a symbolic move in the British efforts to make the military "relationship one between colleagues rather than between enemies."

There has been considerable concern about the safety of the lightly armed monitoring force if fighting breaks out between the Rhodesian forces and the guerrillas. At one base, code-named Foxtrot, in the southeastern part of the country, 48 Commonwealth monitors are all that stand between the Rhodesian forces and about 5,800 assembled guerrillas.

Mugabe has called for the monitoring force to remain in the country for about four months after independence. British sources said the monitoring troops will definitely leave by independence day but that the British government would not exclude the possiblity of providing an advisory force if requested.

Reuter news agency reported from the central Rhodesian city of Gwelo:

An explosion killed two people and wrecked the offices of the Catholic newspaper Moto today, only three days before 2.8 million blacks vote in preindependence elections.

Nkomo held his final major campaign rally in Gwelo today and told reporters he believed the blast was the work of the Selous Scouts, a highly trained section of the Rhodesian security forces.

Moto, which was banned by the former white minority administration in the mid-1970s and resumed publication only recently, supported Mugabe.