How do I get a Chinese iPhone with dual sim?
The obvious answer is to go to China and buy an iPhone there.
Alternatively you can try and find someone you trust to buy the phone and have it shipped to you. If you don't know anyone in China, you can use a service that does this for you. They will give you a local address you enter as the shipping address when ordering from the Apple website. Once they receive the package they will ship it to you in the U.S.
I believe this type of service is called "package forwarding". I don't have any experience with it, and I'm not sure about the legality (probably fine) and costs (service fee + overseas shipping + insurance + import duties, etc).
Here's one I found: https://www.tiptrans.com
Be sure to read the fees, terms, and restrictions. I noticed they don't allow Apple products for some of their warehouses.