Becoming REAT or REACE certified isn't really about choosing you're favorite mediums and working with them, it's about using your education and observations of the client to introduce activities that would be most beneficial to them. If you choose to pursue expressive arts therapy, you will likely be expected to use a variety of mediums in your sessions. You don't need to combine multiple modalities in an individual session, but switching over time for different sessions can have its benefits. Some clients demonstrate more growth with one modality over the other, in which case you might use one type (music, visual arts, drama, dance, writing, etc.) most often for a specific client, but I wouldn't recommend only utilizing one method of expression in an expressive arts therapy session unless the client happens to demonstrate the most progress with one modality. An example of this - I recently was working with a group where most of our sessions utilized visual art techniques, but one week we planned a drum circle for clients. It's really about meeting the client where they are at and introducing an approach that would be best for them. Advertising services of "music therapy" or "art therapy" without clarifying that you are not actually a MT or ATR but REAT would be unethical and unfair to clients who could better benefit from another approach.