Usually – No.

As a specialist, I often have people call and ask if they can do “just DBT” or “just eating disorder” treatment with me. Though there are some instances when, as an outpatient therapist, I might say ‘yes’ to this, it is generally considered unethical.
Exceptions
Couples or Family Therapy

The biggest exception is when you are doing individual therapy as well as family or couples therapy – in this case, you absolutely should have two different therapists! This avoids potential conflicts of interest, splitting, miscommunication, and bias in therapy. It is important in this situation that the couples or family therapist see the couple or family as the client, not one member of the family. That said, especially with teen clients, individual therapists do bring parents in for consult sessions, which are usually not the entirety of the session and are very problem solving oriented and time limited. If the situation would take more than a couple sessions to resolve, it would be most appropriate to turn it over to a couples or family therapist that can assist them on a longer term basis without damaging the integrity of the individual therapy relationship.
Higher Levels of Care
When I worked in higher levels of care, we would treat clients while they were in our facility and ideally transfer them back to their outpatient therapist when they got to the intensive outpatient (IOP) stage so they could have more support but also begin the transition back to normal life at home.
Groups

When I was facilitating groups, I would frequently run outpatient skills groups, usually focused on DBT, where all of the participants were also required to have individual therapists so that they would be able to talk through learning the skills and share things they were not comfortable discussing in the larger group setting. All of the therapists would know what skills were being taught and had training to manage issues that might come up. Sometimes, my individual clients would also be in a group that I ran, and that was OK also.
SPECIALTIES
There are a few specialties, like EMDR, where the therapy is generally considered very short term, and you might see both therapists during that period also, much like the group scenario.
You might have other specialists you see, like dieticians or psychiatrists, who manage other aspects of your care and are part of your ‘treatment team.’ The most important aspect of a team is that they communicate with each other regularly, not just through the client, so they frequently work in the same facility together (like in the higher level of care scenario, though this is often true in outpatient settings that focus on eating disorders). The most important consideration here is communication and coordination of care, which depends on the willingness, time, and ability of all parties involved.
Why would it be unethical?
Here is some info about potential risks and benefits for having two therapists, but I want to focus on the things I mentioned before: miscommunication, splitting, potential conflicts of interest, and avoidance issues. There is also the issue of the code of ethics that therapists are required to follow, and I will address that one first.
Code of ethics
Each type of therapy license has a code of ethics that tells the therapist what is and is not allowed in the therapeutic process and relationship. Most of it is focused on protecting clients. In the NASW Code of Ethics, it is called “dual relationships” and the focus is on potential harm it might cause a client due to conflicts of interest and impaired effectiveness of treatment. Most therapists consider it counterproductive and potentially harmful.
It would be kind of like seeing two different medical doctors for the same issue – a consult is ok, but there is a lot of potential harm if the treatment plans are not the same, and they are likely to be at least somewhat counterproductive. It can compromise the treatment plan of either therapist and could exacerbate the symptoms that brought the client into therapy.
Miscommunication
If your therapists know you are seeing more than one therapist or not, they may have differing perspectives that can be in conflict. They might not both have the same info, because you might not tell both all the same things or forget to tell one and not the other. All of that can lead to confusion and frustration, for the client and the therapist, about what the problem is, what you might need, and what your goals are for therapy. Inevitably, the conversations would overlap and even if you are trying to separate what you talk about, life is usually not that simple and it will come up eventually.
Splitting
The classic example of splitting is when, as a kid, you might ask one parent for something and they tell you what you do not want to hear, so you go to the other parent to see if you can get a different answer, without their knowledge. This can lead to a lot of trust issues, conflict, and frustration. With therapists, even if they don’t know about each other, this can happen internally. This was a reddit thread about how frustrating that can be.
Avoidance
If you have something you need to deal with, chances are it is painful and complicated. Having two therapists can muddy the waters and split your focus in such a way that you might not get anywhere! No one wants that!
You might also just be avoiding conflict. If that is the case, you should know that for most therapists, there is an ethical issue here. We have a contract to ‘do not harm’ just like all other healthcare professions, and we cannot fulfill that obligation if we do not have all of the information. We would WANT to know and address any conflict or confusion you might have. I promise, we would not be offended, hurt, or disappointed if you pick a different therapist. We are all well aware that it needs to be a good fit. You have to trust your therapist in order for the process to work! If you cannot, there is a problem that needs to be addressed.
How should it work?
If you are not getting something you need from your therapist, it is best to talk to them about it.
Most therapists are aware that you might need different treatment approaches at different stages of recovery. Many therapists are not qualified to treat something more complicated like an eating disorder or trauma, and they know it is an ethical conflict, so they will refer to another therapist who is more appropriate. Sometimes, once they are ‘on the mend,’ those clients go back to their original therapist, for various reasons. It is certainly better to talk to them about it instead of ghosting them! If assertiveness or boundary setting are part of what you are going to therapy for (and most people have issues with this of some kind), then it is especially important!
Treatment teams

Treatment teams consist of multiple people in multiple disciplines who are serving different functions (like in a higher level of care facility), they communicate every day and work together. To make a team effective, someone has to be in charge of that communication, and generally that is the client’s individual or primary therapist. If you have providers who don’t know about each other, you could easily be working at cross purposes and making things more difficult than they need to be. I don’t know about you, but that seems like a lot of misplaced effort.
When We Should Refer You to Someone Else
Sometimes, therapists do not like to tell you “you should probably go to someone else for that” but it needs to be said. If you are not getting what you need, and your therapist is not able to help you (as is the case with specialists, dietitians, and psychiatrists), they should refer you to someone else. It is their ethical duty, and someone could lose their license if things go poorly and they failed to do it.
That does not in any way mean that they do not value you as a client, or they don’t like you, or that they do not want to work with you. We have to know our limits also – if I have a client outside of my wheelhouse or who is not a good fit, I will send them to someone else no matter how much I like working with them or think we connect! This part is all about doing what is most effective. What is most effective is what is in your best interest, not the therapist’s.
Every person needs different things at different times in their life, and therapists know this even better than most. Our entire job is about helping people, and no one is good at working with all kinds of clients. No client is right for any old therapist. When you find the right one, there should be enough trust for you to say “this is my goal, this is what I need right now” and for you to find an appropriate way to get that. Goals
change. Therapy/therapists need to change with it.