I'm in a similar situation, having switched T's because I didn't think my ex-T was helping anymore, but also to help me deal with transference (and likely countertransference) with my marriage counselor (who I am still seeing).

Shortly before I started seeing new T, I found out he used to work in the practice with ex-T and MC! So that made it particularly awkward for me. I was originally going to try keeping ex-T and MC anonymous, but I realized if he worked with them and is still in contact with MC, he'd probably figure out at least who MC is if I shared all the details. So I ended up e-mailing him before our first appointment, telling him who I was seeing, and asking if he still felt comfortable seeing me to address some issues with them. He said he did and stressed that he would keep everything confidential, not just for ethical reasons, but because he understands how important that is.

The first couple sessions, I felt awkward going into much about ex-T and MC, but then by the third session, I started trusting him more (something about him) and just started pouring things out regarding MC and all the stuff going on there. He seemed to really listen and be fairly objective, though he did make a few comments because he knows MC--like when I talked about how much he self-discloses, T was like, "Yeah, he's an open book." (The most awkward was when he referred to MC's late wife by her first name and said, "Has it been that long already?" when I mentioned her passing last December...)

I was afraid he'd be really defensive of MC, not only because he's a fellow T, because also because he used to work with him and seems to be friends with him. But it hasn't really seemed that way. He has provided alternate explanations for some of my interpretations of MC's actions, but, I mean, he does that for my H and other people, too (Like, "maybe they're doing that because of x rather than y.") Like, when MC called me in response to an e-mail at nearly 10 p.m. one night, which I found a bit odd because of the late hour, T said he doesn't get the sense MC has the best awareness of time (seems accurate, in my experience!), so he probably didn't even realize how late it was. At the same time, I don't feel he's defending him so much. Especially in terms of things like boundaries, disclosures, etc.

Granted, my situation is different because I'm more looking for help from T in detaching from MC and am trying to figure out his role in my attachment (rather than being sure he's caused me harm). But as part of that, I've had to be critical of one T to another.

I think a good T should be able to be fairly objective and not automatically defend your ex-T because they share a profession. You could start out with new T by saying something like, "I feel I've been harmed by ex-T and need to work through some of that with you. So I'll have to be critical of ex-T to you, and feel sort of weird about that, because you're also a T. Do you feel comfortable working with this topic?" That way, you get the discomfort out in the open and can see how the T responds. Most likely, it will take multiple sessions to see if it will work out, but if she (or he?) seems immediately uncomfortable with the concept of you criticizing another, then you'll know you should probably find someone else who is comfortable.

OK, that was probably way more information than you needed to know, but hopefully it was somewhat helpful!