KENYA and Zaire are unlikely twins, but both are deeply stained with corruption. Both boast despotic rulers determined, Canute-like, to stay the tides of democratic change. Both harbor regimes which have long been backed by the United States.The peoples of both deserve better from the US, possibly such determined steps as sanctions and adverse travel advisories. Kenyan President Daniel arap Moi for years has refused to respect human rights or permit expressions of dissent. Late in November, his police busted up the most recent opposition rally, detaining a dozen advocates of multiparty democracy. Earlier, he ended what limited press freedom existed in Kenya, jailing the editor of a legal publication and banning magazines. Some protesters were released from prison after American and world complaints; others were locked up. Even the public and private efforts of US Ambassador Smith Hempstone Jr. have failed to halt the latest waves of repression here. Instead, Mr. Smith has been condemned by President Moi. Two coup attempts against Moi's reign have failed during the past decade, and the US and other governments have reduced, but not completely cut off, aid. Indeed, during the Gulf war Washington was anxious not to anger Kenyan officials, since the Indian Ocean port of Mombasa had some standby utility. Washington has been reluctant to move against one of the more prosperous and more loyal regimes of Africa. Just as it was slow to appreciate that President Mobutu Sese Soko's days were numbered in Zaire, so it has not wanted to turn against another ally. But the extent of President Mobutu's misrule and corruption is now well known, thanks to Zaire's descent into chaos this year. Kenya's equally venal kleptomania has also come into view. Robert Ouko, Kenya's foreign minister and a Luo from the far western part of the country, was killed mysteriously in early 1990. At the time, close observers of Kenya surmised that the slaying had been arranged. Now, thanks to the investigations of a British detective, Moi's closest colleagues are prime suspects. In mid-November, Moi dismissed Nicolas Biwott, his strong right-hand man, from the Kenyan cabinet. Earlier he had sacked Hezekiah Oyugi, the head of Kenya's notorious internal security operations. Mr. Biwott and Moi are both Kalenjin, one of the country's smaller ethnic groups. It seems clear that Mr. Ouko knew that Moi and Biwott were corrupt. He seems to have resented the extent of their skimming of public funds, some of which were intended for industrial development and others for the alleviation of poverty. Biwott was known as the president's chief aide and counsel. He was feared by Kenyans for his ruthlessness and his greed. He was a leading defender, too, of the one-party state and of the small tribes of Kenya against the larger Kikuyu and Luo. In both Kenya and Zaire, the US has the influence and the wherewithal to encourage more punctilious respect for human rights and multiparty democracy. Since Kenya relies upon the West for financial assistance and for a steady flow of tourists, approaching two-fifths of its foreign exchange earnings, Washington ought to be able to persuade a severely weakened Moi to permit public discussion of democracy. Last week Western nations aiding Kenya said financial help would be continued only if Moi moved toward fuller political participation. This week he responded by ending Kenya's ban on political parties in addition to his own. But opponents were skeptical about whether free elections would, or could, be held with Moi still in power. Washington rightly worries about instability in Zaire after Mobutu's departure. But Kenya after Moi is less problematical. Kenya, unlike Zaire, works better than nearly all the countries of the continent. Its infrastructure and educational systems are intact, its treasury reasonably full, and its people much more united. Moreover, there are many well-trained Kenyans who are thirsting for the kinds of unfettered political participation that are now becoming more common in other parts of Africa: in Zambia, in Botswana, in Namibia, in Senegal, in Togo, and soon in Nigeria and Angola. The murder investigation will doubtless come close to the Kenyan presidency, and Moi, like Mobutu, may need to beat a retreat. Washington has always known how corrupt and unprincipled both rulers were; now is the time to help save Kenya, at least, from further disarray.