You should be fine to use both. Given that balm acts as a hold, you'd want to apply it after you've let the butter soak in a bit. And of course, it's best to wash your beard before all this to get rid of any dirt. That said, I imagine that the texture of the balm will be the most noticeable.

They both help with beard health, but the main difference is how they affect the look and feel of your beard. Balm will help hold it into shape, whereas butter makes it feel softer.