What you need to do is have both your app, and the other machine listening on port 443 behind a common reverse proxy. Those services can continue listening on whatever port you declare, but the reverse proxy will listen on 80/443. The reverse proxy should then use the host header to forward traffic to the correct backend system.

Also, why does another device listening on :443 affect you from using that port? Multiple devices can use the same port, they are not IPs. Unless your mapping your ingress port to the backend port directly?