Noun
A joint regent or ruler.
Source: Webster's dictionaryDuke of Austria and Styria (with his brother Leopold I) from 1308–1330; officially co-regent of emperor Louis IV since 1325, but never ruled. Source: Internet
Joseph was also made co-ruler or co-regent with his mother. Source: Internet
Joseph himself often threatened to resign as co-regent and emperor, but he, too, was induced not to do so. Source: Internet