From what I've read about Commodus, he probably decapitated his twin! I also find it rather suspicious that every single one of his other brothers also died in childhood, whereas almost all of his sisters lived to adulthood. Speaking of the baby's "head" on that coin, I take it that nobody thinks the raised circular dot between the two boys, visible on almost all coins of this type, is anything other than a so-called "compass dot"?
Yep! Just a compass dot. Barely visible compass dot (just above the baby on the right's knee): Big compass dot -- to match the empress's ostentatious stephane, I guess: