Joshua Nkomo and Robert Mugabe stood together outside the smoking ruins of Nkomo's bazooka and bullet-shattered shell of a home in the fashionable residential Longacres area of the Zambian capital.

"What the enemy wants to achieve has not been done," Mugabe shouted out to a crowd of sympathetic Zambian onlookers, referring to the apparent Rhodesian attempt to kill or kidnap Nkomo during a daring commando raid on the Lusaka-based guerrilla leader's home April 13.

"Together we are determined to carry the battle to the finish," said Mugabe, who lives in the Mozambican capital of Maputo.

It was a rare public demonstration of togetherness for the two coleaders of the Patriotic Front, the shaky alliance of about 40,000 guerrillas fighting first to overthrow the white-minority Rhodesian government and now the new black-led one to take office later this month under Bishop Abel Muzorewa.

For years now, Rhodesia's black nationalist guerrilla leaders have been moving apart as they fight for the same cause from opposite sides of the country under different leaders and organizatons with separate backers in Africa and the Communist world.

To the growing despair of Africa, the Soviet Union, Cuba and particularly the five african front-line states of Zambia, Tanzania, Angola, Botswana and Mozambique, the specter of a second civil war, between the two large guerrilla armies under Mugabe and nkomo, looms ever larger as their war expands inside Rhodesia and both step efforts to gain military and political dominance inside the country and diplomatic supremacy outside.

The possibility of internecine confrontation between nationalist factions is fast becoming black Africa's newest political nightmare. It could affect whether a number of moderate African states decide to stick with the Patriotic Front or switch to recognize the newly elected Muzorewa-led government in Salisbury.

Evidence of dissension between Mugabe and Nkomo is gist to the publicity mills of those in the West arguing that Washington and London should recognize the Muzorewa government if only to avert an even worse civil war than the one going on between Rhodesia's moderate and radical black nationalists.

The two Patriotic Front leaders met for three days in Addis Ababa, Ethiopia, earlier this month, and said the meeting led to "a luminous and heroic landmark" in the nationalist struggle. They did not announce a merger of their political parties, however, and rejected a front-line proposal that they set up a provisional government that their backers could recognize.

On the other hand, they agreed to set up a new coordinating council to lead the Front and to revive a joint defense council to coordinate war tactics under a unfied command.

Basic differences remain, however. Nkomo insists that political unity must precede a fusion of their armies, while Mugabe argues just the reverse. Mugabe bases his argument on the fact that the war effort is of paramount importance at this time, while Nkomo reasons that yet without political unity, two armies will never work together.

Yet arguments of both leades probably reflect better the relative strengths and weakness of their respective organizations. Mugabe's Zimbabwe African National Union accounts for the vast majority of the 12,000 guerrillas inside Rhodesia and controls considerably more territory and population.

Nkomo's Zimbabwe African Peoplehs Union has superior experience in political organization and could outmaneuver Mugabe and his lieutenants in a unfied political alliance.

Nkomo, 61, ranks as the grandfather of Zimbabwean nationalism and is a wily old politician.

Officially, both Mugabe and Nkomo heatedly deny the possibility of a civil war between their well-armed forces, each numbering at least 20,000. They say that talk of such conflict is the work of the "imperialist" press seeking to sow division between their factions.

But few outsiders - African, Western or Eastern - believe this any longer, and Tanzanian President Julius Nyerere has publicly expressed his fears about the increasing likelihood of a second civil war, between the two wings of the Patriotic Front.

The history of Rhodesia's black nationalist movement is replete with factional infighting, power struggles and alliances forged and broken again and again.

Tribal conflicts, personal ambitions, differing political strategies and ideologies and sheer pride of organization have all helped divide the nationalist movement since it began with the formation of the African National Congress under Nkomo in 1957.

Today, the internal black leaders who are about to take over Rhodesia's first black-led government are divided from the leaders based outside the country both by ideological differences and the ever-present struggle for power.

Bishop Muzorewa, slated to become Zimbabwe-Rhodesia's first black prime minister, is an open exponent of capitalism and derides socialist experiments in Africa as failures.

Mugabe, 51, is an avowed Marxist and Nkomo is a socialist of undeterminable ideology.

what separates the internal and external nationalist leaders above all is the feeling among the latter that Muzorewa is reaping all the fruits of their labor on the battlefield that forced Rhodesian whites finally to hand over power.

The feud between Mugabe and Nkomo also involves different ethnic constituencies, war strategies and ideological convictions. Mugabe's popularity is rooted in the majority Shona people while Nkomo's is among the minority Ndebele, although both have some supporters within both ethnic groups.

Mugabe's National Union has adopted "Marxism-Leninism-Maoism" as its ideology and a Maoist war strategy of first conquering the countryside and then storming the cities. Nkomo, under appraent Soviet advice, has concentrated on building up a more conventional army and using sophisticated weapons such as the SA7 surface-to-air missiles that have brought down two Air Rhodesia passenger planes.

Nkomo also seems more interested in the cities than the countryside, partly because the western area of Rhodesia where he is strongest is sparsely populated and partly, no doubt, because the capital is important in any power struggle.

Throughout the Rhodesian struggle, Prime Minister Ian Smith has been a master political manipulator of the personal rivalries and divisions among the nationalists. He has built up one leader or faction after another, playing one against the other.

At least part of the division between Mugabe and Nkomo can be traced to Smith's repeated successes in tricking Nkomo into negotiating with him. After holding official talks with Smith between December 1975 and March 1976, Nkomo met secretly with him here in August without informing Mugabe beforehand.

But the front-line leaders must also bear some of the blame for creating a deep fissure in the guerrilla movement. Zambian President Kenneth Kaunda always has shown a strong preference for Nkomo, stemming from their close friendship in the early years of the nationalist struggle against the British colonial power in Zambia and Rhodesia.

On the other hand, Presidents Julius Nyerere of Tanzania and Samora Machel of Mozambique have taken an equally partisan stand in favor of Mugabe and his National Union, which was founded in 1964 in Dar es Salaam at the Tanzanian leader's urging.

The Communist world also has been split in its support of the nationalists, Until recently, the entire Soviet Bloc was helping only Nkomo while China backed Mugabe.

Outside observers are skeptical that the two will unite, even if they formally accept the plan under pressure, because of the past history of intense, Bloody feuding within the Front. CAPTION: Picture 1, ROBERT MUGABE . . . holds military edge; Picture 2, JOSHUA NKOMO . . . more political experience