I've never liked having two Seleukid armies either. Unfortunately, it isn't possible to have enough Seleucid units to accurately portray the army's diversity and numerical superiority without having two. Starting the armies together was actually sort of a cheap trick of mine to encourage the 1.2 AI to fight. In my first version I had them farther apart, but the AI-controlled units only wandered around aimlessly. It is a lot more fun on 1.5. The Seleucid phalanx tends to be more cohesive and the cavalry aren't quite as stupid.