Absolutely you should do it. I think, if I'm understanding correctly, that this conversation is a version of "this therapy isn't working out for me" and that your desire to have him contact your old T is your perception of how to fix the problem.

I've had a variation of this conversation more than once with all of my T's, and IME I think they relish it, and are not alienated by it. I think they see it as you taking the reins of your own therapy and being willing to put it out there with the feedback that you need something different from them. It is progress even to be politely confrontative about this issue, but I have approached it sloppily as well as probably with some amount of sophistication, and all of them have responded warmly and enthusiastically to the feedback that they weren't giving me what I needed right then. It has always been a turning point in therapy.

You could say "I want you to contact and talk to my old T about how you could be a better therapist to me", but I'm not sure that's really the issue. The issue is that you haven't yet built the kind of relationship with him where you can tell him what you need from him-- I think, anyway. I doubt it would be helpful for your therapy for him to talk to your old T, and even asking the question this way seems to be to be a backhanded way of getting someone else (your old T) to be responsible for making your current therapy work. I think you are the one responsible for making your therapy work.

So why not say, I'm not getting what I want out of this therapy? Maybe he'd ask you for ideas for what you want or how the two of you can change things. You could say, talk to my old T, I don't think that's an illegitimate suggestion or recommendation, and you could have a conversation about that.

I have been repeatedly surprised by how much my current T (and my last therapy was 15 years ago, so my memory is not as good for it) really seems to relish the feedback that I give him about what is working for me, and it's about 80% negative. So I don't think you have to be worried that your T is somehow not capable of hearing this or dealing with it.

I also think that this is kind of an intimate relationship thing, too. I have said to my H, this (whatever he's doing related to a specific thing) is not working for me. We have to do something different. Now "be different" is not a terribly useful thing to say to someone, but it at least starts the conversation and it can lead to the other person understanding HOW to be different in a concrete way.

So I think your issue here is a very important and central one, that is not only relevant for therapy but for intimate relationships more generally.