Every once in awhile I get a call from someone who wants to see me in addition to the therapist they are already seeing. In theory it sounds pretty good. You can work through one issue with one therapist and another with the other therapist. You also get two different perspectives and can pick and choose what works best from each. On the surface it seems ideal as long as you can afford the fees of two therapists.
However, generally there are more reasons not to see two therapists at the same time and some exceptions to the rule as well. Ethically, it is not appropriate for therapists to treat a person for individual therapy while they are being treated individually with another therapist. It would be similar to seeing two cardiologists for the same heart condition.
Both doctors may be well qualified but they each have their individual approach towards treatment. If you combine them the results could be deadly. Now, combining two therapists may not be deadly but it can compromise the treatment plan of either therapist and could exacerbate the symptoms that brought the client into therapy.
The exceptions to the rule include interviewing therapists or looking to change therapists...
Why Can Having Two Therapists Be A Bad Idea? by Julie Cohen
"Too many cooks in the kitchen." vs. Benefits of synergy
More insight vs. "Insight does not equal behavior change"
(With the OCD I have, for example, trying to seek more knowledge/insight is, at least eventually, more part of the problem than the solution.)
Whether it is the best plan or not, who are they to say it can't happen.
Why not tell the client the "master plan"?
It seems to me more of a situation of informed consent -as in informing the client = if you work with two therapists, then each one may be less effective - then the client can choose whether to take that risk or not. It is, after all, the client's time and money and life.