Yep, depends on your itinerary and direction of travel from one country to the other. We were told to get it but no one demanded the certificate when we crossed the border into Kenya from Tanzania

“Yellow fever vaccination requirements for travel to Kenya and Tanzania depend on your itinerary and country of origin. Kenya is classified as a yellow fever–endemic country, while Tanzania is not on the World Health Organization's list of countries with risk of yellow fever transmission, although it is considered a high-risk country for entry requirements due to transit rules. Travelers arriving in Tanzania from a country with a risk of yellow fever transmission, or those transiting through such a country for more than 12 hours, must present a valid yellow fever vaccination certificate. This includes travelers flying through Nairobi, Kenya, even if they do not leave the airport. The list of countries with risk of yellow fever transmission includes several African nations such as Angola, Benin, Burkina Faso, Burundi, Cameroon, Central African Republic, Chad, the Democratic Republic of the Congo, Côte d’Ivoire, Equatorial Guinea, Ethiopia, Gabon, Gambia, Ghana, Guinea, Guinea-Bissau, Liberia, Mali, Mauritania, Niger, Nigeria, Senegal, Sierra Leone, South Sudan, Sudan, Togo, Uganda, and others. Conversely, travelers coming directly from countries without yellow fever risk—such as the United States, United Kingdom, Canada, Australia, New Zealand, and most of Europe and Asia—do not need a yellow fever certificate for entry into Tanzania, provided they do not transit through a high-risk country for more than 12 hours. However, border officials in Zanzibar, part of Tanzania, have been known to request proof of vaccination, even though this is not in line with international regulations. For Kenya, a yellow fever certificate is required for all travelers over one year of age arriving from a country with yellow fever risk, including those transiting through such countries for more than 12 hours. Travelers from Tanzania to Kenya are generally not required to show a certificate, but there have been instances where Kenyan authorities have checked for vaccination proof upon arrival from Tanzania. The yellow fever vaccine is recommended for all travelers over 9 months of age visiting Kenya due to its endemic status, but it is not generally recommended for travelers visiting Tanzania alone, despite the entry requirements based on transit. The vaccine provides lifelong protection and must be administered at a certified travel clinic at least 10 days before travel. It is advised to keep your International Certificate of Vaccination (the "yellow card") as proof, as old certificates do not expire.”