I've always considered that the sasanquas are better suited to hedge training, rather than the japonicas. That opinion is based simply on the differences between the growth habit, and even the speed at which the two species grow.

I assume that you know that these plants grow very slowly, but can get quite bulky as they age. I'd not plant them any closer than 3 feet (3 ft. centers) and begin training them now. Be sure to plant them so that they have room to develop horizontally, as well.

Japonicas tend to grow in a tree-form fashion, so pruning them carefully will be important. Hedge plants should be pruned to be slightly narrower at the top than at the bottom so that the lower part of the plant receives decent light. That is, unless you visualize an allee of Camellia japonica trees, trunks exposed, with the canopies growing together. What a lovely show that would be!