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