How to deploy Next.js and Express servers on two different ports concurrently on Heroku
Don't do this. It's a better idea to keep the front-end and back-end (API) completely separate, on a coding point of view but also on a hosting/infrastructure point of view. Read about "monolithic vs microservices".
So you can create 2 Heroku apps, one for Next and one for Express. You will have to link each other apps by setting the correct URLs and perhaps also configure CORS on the API.