Question: I am on heroku and have the Redis to go add-on, I am using it for store my translations and now I want to use it for Resque background jobs as well.

Can I use the same instance or should I configure another one?

Answer: 
Although you can use a single instance for multiple purposes, I'd generally recommend setting up a separate instance - resource usage is more related to the amount of data than the number of instances, and if you later need different settings for one app, splitting them can be non-trivial. 

Security and troubleshooting also need to be considered - you can restrict an application to accessing a specific port, but there aren't any lower level permissions available.

The choice may be a little different if you are using an overpriced hosted service that charges by instance rather than usage, but having the additional flexibility is probably still worthwhile.
I can't speak specifically to Heroku's implementation of Redis, but in most cases there's nothing preventing you from using the same database. In fact, Resque assumes you might and uses the the [redis-namespace](https://github.com/defunkt/redis-namespace) gem for all of its redis keys. You could use the redis-namespace gem in your own application as well to keep every type of cache in a different namespace.