While it is of course more convenient to use african troops in Africa and iberian troops in Iberia, if you prefer historical accuracy, you may consider doing it a different way. The Carthaginians used to deploy iberian troops as garrisons in Africa and vice versa. The reason for this is, they thought it less likely for the garrison to join a possible uprising if the soldiers are not native to the region. As said, this applies to garrisons, an offensive army would of course use anyone available.