Complex sentence: They advanced through their careers at the same pace and were going to rule together; however, they died young, forcing Augustus to draw another line of succession, which is why in AD 4, Augustus adopted Tiberius on condition that Tiberius adopt Germanicus.
Split into simpler sentences: They advanced through their careers at the same pace and were going to rule together; however, they died young, forcing Augustus to draw another line of succession. Which is why in AD 4, Augustus adopted Tiberius on condition that Tiberius adopt Germanicus.