web-server.conf

server {
    listen 80;
    location /.well-known/acme-challenge/ {
        root /path/to/certbot;
        try_files $uri @redirect;
    }
    location @redirect {
        return http://xmpp-server.example.com$request_uri;
    }
    server_name example.com www.example.com;
}

xmpp-server.conf

server {
    listen 80;
    location /.well-known/acme-challenge/ {
        root /path/to/certbot;
    }
    server_name xmpp-server.example.com;
}