This is finally possible to do properly since 1.15.2. See the information here.

your nginx.conf add a block like this (outside the http block):

stream {
    upstream http {
        server localhost:8000;
    }

    upstream https {
        server localhost:8001;
    }

    map $ssl_preread_protocol $upstream {
        default https;
        "" http;
    }

    server {
        listen 8080;
        listen [::]:8080;
        proxy_pass $upstream;
        ssl_preread on;
    }
}

Then you can create your normal server block, but listening on these different ports:

server {
    listen 8000;
    listen [::]:8000;
    listen 8001 ssl;
    listen [::]:8001 ssl;
...