The distance between your horses in relation to the pole is determined by the placement of the swingletrees (aka whiffletrees -not sure I spelled that right- or sinlgetrees as many Americans refer to them.) Usually in combined driving horses are always kept up tight and close to the pole. That is because the placement of the swingle trees and the width of the swingletrees is pretty narrow. This makes driving through obstacles and cones more accurate and tight. Draft horse swingletrees are generally wider and set further apart which will mean the horses need to be further apart.
So in short, you're horses should be straight in front of the middle of the swingletrees of whatever vehicle you are driving. To keep the horses in the correct positions, you'll need to adjust your crossover rein (aka coupling rein) along with the pole straps and traces. American light and heavy draft vehicles usually have a yoke between the horses. A yoke is also going to affect where the horses are placed by the pole. Here is a picture of my Saddlebred and Saddlebred/Friesian cross with a light draft American vehicle and a yoke between them:
You can see that by the placement and width of the swingletrees along with the yoke, when put to this vehicle my horses are wider apart than when put to a marathon or dressage carriage. If you need to bring your horses closer together (if they are pulling apart from each other, bulging over their shoulders, or pulling towards the outside of their collars -these horses sometimes rub the hair off the outside shoulders a bit) you need to shorten your pole straps a couple holes and probably bring your traces tighter by a hole. You can also bring the coupling rein tighter a hole, but most of the time I find that correcting the fit of my pole straps and traces is the most effective way to bring the horses closer to the pole and then drive them with light encouragement forward and towards the pole from the whip. Gentle touches on the outside shoulder with the whip... no whacking or smacking! The most important thing to look for is straightness. You're horses should be straight pulling the carriage with even tension in both traces when everything is adjusted correctly. In case you need some help on the length of pole and length of traces, below is my video on how to put to a pair, which also includes a little bit of adjustment of pole straps and traces:
I hope this can be of a little help in your driving of pairs and their placements to the pole!