Besides the marching speed or the place on the battle map, you can also order them to attack a certain enemy. Two examples:

1. I attacked three stacks with two stacks. I placed the reinforcements on my left flank on a defensive stand and used my stack to crush the first enemy stack. Next I used my cavalry to defeat a small second stack and put the reinforcements on offensive. They started to pursue the remnants of the first two stacks when I noticed that the third stack was not in front of me but on my right flank . I ordered my reinforcements to attack the third stack from behind and attacked with what was left of my own stack. The third stack was crushed between two stacks and I won the battle.

2. I was besieging a rebel village with New Spain. The rebels sallied just at the moment when an army of my allies was passing by. I was able to control the allied army and attacked from two directions.

I like the new feature. I would have preferred to completely control the second stack but it works OK for me.