Managed Redis and multiple app platform instances

I have two app platform Django instances for a staging and production environment. Both have celery and celery beat. I’m paying for two Managed Redis instances. I’d like to pay for only one.

I’ve seen that you can use multiple databases in the same Redis instance (i.e. 'redis://redis-cluster-url:6379/1' or 'redis://redis-cluster-url:6379/2').

But when you attach a DB in app platform, you access the environment variable with something like this: ${dbname.REDIS_URL}.

Is it possible to change the database instance in Managed Redis (/1 or /2)? Is there a way to change the database instance in app platform? Or do you have to do the full connection string with the database id instead?