The beauty of starting trees in a root pruning container system is that you can do 1 nut per container and then cull those trees that don't perform well. Some nuts will just produce weak trees. When you have two in the same container, they steal resources from each other and neither tree does as well as it would if potted along. For folks who direct seed, it is a common practice to put more than one nut in the hole that you tube and protect. With direct seeding, you don't know if the nut will germinate. You simply pinch off all but one after they start growing. When using the root pruning container system, you cold stratify (if needed for the particular species) and can germinate the nut before you plant them so you know the nut is germinated. If you have enough 18s to cover the number of nuts, you can just plant them in the 18s without germinating them first.
Imagine what that tree Teeder posted if all that wood was in a single tree!
Thanks,
Jack