I juggled two within an ~2.5 year period of time. One was a psychoanalyst, the other eclectic. First I saw one for several months, then a small break, then the other one for several months, and in the end I had irregular sessions with both here and there, while simultaneous email contact with both, all sorts of things. I was 100% open with both about all this. One told me in the beginning that seeing two Ts at the same time is typically considered a 'no' by Ts, but he was open to whatever worked for me. They both criticized the other, and my involvement with them, at times. I did not mind.

Basically, they say whatever they want, and perhaps it is interesting to listen to their considerations, but it is the client who employs them and should be in charge of what we want to do with our therapy.

Maybe just ask them individually what they think about your seeing another T as well, before declaring it? But, again, this should not be their decision, they are independent service providers.