Chestnut trees can be kind of tricky. There are both edible and inedible types. I looked up the Dunstan, they are the edible type. I have no idea if the Chinese you got are edible or inedible. The seed coverings have some extremely sharp spikes. Use leather gloves to handle them. Those things are dangerous.
They can also grow into huge majestic trees (40 to 60 feet high maybe) or more of a tall shrub. If the Dunstan is grown for seed I'm pretty sure they are the tall majestic type, not sure how a graft might affect their growth habit. Again, no idea on the Chinese. Hopefully the people at the nursery can help you with some of this.
Dad planted an ornamental Chestnut. The nuts were inedible and it made a "tall shrub" maybe 20 feet high. It was an attractive tree but those seed coverings were dangerous.
I understand you are running out of room. I'm not sure of the growth habit of that Chinese but at ten feet separation they will fight each other for sunlight. The canopies will probably grow into each other and the trees might become sort of tall and narrow. If one grows a lot faster than the other it might sort of shade the other out. If you walk through an old growth forest you will seldom see trees as close as ten feet apart. It is what it is, try it and see what happens. It will be years before you know.
From what I read Chestnuts are not self-pollinating. You might chat with the nursery about whether or not those tow will pollinate each other.