It would be a bit of a hassle, but if you want to make it look like there's more space between them at the start then you could set up just one army on one side of the field and take pictures of it from the far side, then later set up the other army by itself and take it's pictures from the other far side. If that makes any sense. It would increase the apparent space between them for the establishing shots. You can put just a few of the very front line just in front of the camera to add additional sense of depth.
Photo story photography isn't the same as setting up a display diorama. On a display you can see everything at once so you need to get it all in there. For a photo shoot you just need what's inside the camera frame at any given shot.
