LotR -- Book 5 - Chapter 10 - The Black Gate Opens

[William Cloud Hicklin]

Dividing your force between two hills: not optimal, but if the hills aren't very big, too small to get your whole force on just one, then you have a choice: deploy them as Aragorn did, or deploy them in a continuous line with the flanks on the hills. There is merit in that, but on the other hand, it creates a weak spot in the middle of the line where the enemy can break through. Moreover, when you are grossly outnumbered, you want to present as small a front as possible to the enemy, so he can't bring his numbers directly to bear (think Thermopylae).

Of course Aragorn's little army was still doomed barring a miracle (which happened), but if the idea was to buy as much time as possible then two schiltrons on elevated ground make a lot of sense.

What makes ZERO sense is the movies' suicidal banzai charge, which would have been wiped out in five minutes.