They will be the first Africans to receive the treatment, while authorities in Spain said a 75-year-old priest had died of the disease.
The death toll from the worst ever outbreak of the highly contagious disease has climbed to 1,013 since it was discovered in remote southeastern Guinea in March, according to the World Health Organization (WHO).
(READ MORE: Africa needs to look inward to tackle Ebola)
It said ZMapp doses were very scarce, raising ethical questions of who should have priority.
Spanish authorities said a 75-year-old Spanish priest who contracted Ebola in Liberia had died. The government had announced on Sunday that Miguel Pajares, the first European infected by the strain, would also be treated with ZMapp manufactured by California-based Mapp Biopharmaceutical.
In addition to Pajares, ZMapp has already been administered to two U.S. aid workers. The U.S. citizens are now in a hospital in Atlanta, Georgia, after being medically evacuated and have shown some signs of improvement.
The virus - one of the deadliest diseases known to man - has spread to four African countries, infecting a total 1,848 people, according to the WHO, which has branded the outbreak an international health emergency.
The epidemic in one of the world's poorest regions, where crumbling healthcare systems are unable to cope, has opened an ethical debate on the use of trial drugs on humans. A WHO medical ethics committee was due to announce its findings on Tuesday, including on the sensitive issue of who should receive priority for the limited supplies of the drugs.
(READ MORE: Liberia president apologises for high death toll of health workers)
With medical staff lacking the equipment and training to tackle the first outbreak of Ebola in West Africa, more than 60 healthcare workers have died and dozens more been infected, severely hindering countries' ability to cope with the disease.
Information Minister Lewis Brown said the Liberian government had received written consent from the two doctors - who he identified as Zukunis Ireland and Abraham Borbor - for the treatment, which has not been fully tested in humans.
"The drug maker could not export...the drug without the approval of the FDA so our authorities approached the FDA and received specific approval for the treatment of these two doctors," Brown told Reuters by telephone.
SCARCE SUPPLY OF DRUG
He said the drug was expected to reach Liberia within the next 48 hours. A statement on the Liberian presidency's website had earlier said U.S. President Barack Obama had approved export of ZMapp but the minister said this was incorrect.
A spokesperson for the U.S. Health and Human Services (HHS) Department said U.S. authorities had simply assisted in connecting the Liberian government with the drug's manufacturer and followed procedures for the export of pharmaceuticals.
Mapp Biopharmaceutical said on Monday its supply of the drug has been exhausted, after the company provided doses to a West African nation, according to a report published by the Wall Street Journal.
The Liberian presidency statement said the head of the WHO, Margaret Chan, had authorized the dispatch of additional doses of the experimental drug to Liberia, but Minister Brown said it was not clear if this was true.
A WHO spokeswoman said supplies were "very scarce".
"They have less than a dozen (doses) of it," spokeswoman Fadela Chaib told a briefing. "It is very important to discuss who should get it...and if it is ethical to use it."
The WHO has said the epidemic will likely continue for months as the region's healthcare systems struggle to cope and has appealed urgently for funding and emergency medical staff.
(READ MORE: Nigeria injects 1.9 bln naira into Ebola battle)
Ivory Coast, the economic powerhouse of French-speaking West Africa, on Monday banned air travellers from the three countries worst-hit by the Ebola outbreak and ordered its flagship carrier Air Cote d'Ivoire to cease flights to and from them. Ivory Coast has not registered any cases but is seen as vulnerable given its shared borders with Guinea and Liberia.
Highly contagious, Ebola kills more than half of its victims. It is believed to have been transferred from fruit bats to humans in Guinea late last year and then spilled over in neighbouring Sierra Leone and Liberia.
The virus has since spread to Nigeria via a passenger from Liberia who collapsed in the busy Lagos airport in late July and later died. Nigeria now has more than 10 confirmed Ebola cases, its health minister said on Monday.