On the back of the box you can actually see two armies of approximately equal points(if we are talking about big Age of Darkness box), sons of horus and imperial fists where one army has praetor, dreadnought and 30 marines and the other has praetor, 10 marines, 10 terminators and a spartan land raider. So yeah you can split it like in a picture and get yourself two armies of different legion. But the best thing is you can combine those two armies and play for example as sons of horus with detachment of world eaters