LONDON — Ever since colonial cartographers drew their common frontier across the Zambezi River, Zambia and Zimbabwe have shared an uneasy destiny: landlocked nations, onetime adversaries and now mirror images of political uncertainty.

In Zambia on Tuesday, a split in the governing Patriotic Front after the death in October of President Michael Sata deepened as a faction supporting the acting president, Guy Scott, Africa’s only white leader, chose a 44-year-old economist, Miles Sampa, as its candidate to run in elections in January.

Another faction had already appointed Edgar Lungu, the defense minister, to the same position.

In Zimbabwe, also on Tuesday, the governing ZANU-PF party of President Robert Mugabe prepared for a congress that is expected to rewrite the party’s profile, endorsing Mr. Mugabe’s 34-year grip on power and anointing his wife, Grace Mugabe, to high office while sowing uncertainty over a choreographed contest to become his heir apparent.

In both countries, the maneuvers among the elite risk alienating ordinary people and political figures such as Rugare Gumbo, a former ZANU-PF spokesman, who was purged last month as Mr. Mugabe moved against supporters of the country’s vice president, Joice Mujuru, a former guerrilla fighter once seen as his likely successor.