Diarchy
Diarchy (or dyarchy) is a form of government where two people are usually the heads of state. The word comes from the Greek δι- "two elements" and ἀρχή, "rule" (from ἄρχω; -αρχία is a derived suffix). The term duumvirate is a hyponym (from Latin duumvirātus, "male diarchy") but some people use it erroneously for women.
Historical examples Diarchies are Sparta, The Roman Republic and at one point India
Modern diarchies
Diarchies are very uncommon in the modern day. However, there are a few that remain.
The Principality of Andorra is a small country in Western Europe. The two heads of state are the Co-Princes of Andorra. As per a historic treaty, the Co-Princes are the Bishop of Urgell and the Ruler of France. The current Co-Princes are Emmanuel Macron and Joan Enric Vives Sicília.
The Kingdom of Eswatini is a country in Africa. The King and his mother are the joint heads of state.
The Republic of San Marino is a small country in Southern Europe. It is the oldest republic in the world. The two heads, the Captains Regent, are elected by the legislature every six months.
