Nice coin! I have always been fascinated how the histories of the time flatly contradict one another; one source saying Vaballathus and his mother were paraded through Rome then pardoned and allowed a pampered retirement, while another says he succumbed to plague en route.
Rome actually has an interesting array of double portrait coins of client kings--Edessa had the Abgar line, Bosporus had an impressive lineup stretching from Tiberius all the way to Constantine. Can't forget the Herodian dynasty, even if they were forbidden to place the human likeness on their coinage.
Also interesting is that the legend here VABALATHVS VCRIMDR establishes his status as a client king under Aurelian: Vir Clarissimus Rex Imperator Dux Romanorum