I think .. you need to work out what you want from talking to t9 about t8; and also if you really want to return to t8 or not and if so if it's t8 you need to talk to more than t9 about t8
As far as broaching the subject, it's your therapy and they should be able to handle that in a professional manner but t9 can't really tell you what was in t8's head or explain their actions

I've talked to sT about an old therapist I had; and it helped being able to talk but it was also annoying because he made it clear it was my interpretation of what happened and he was only getting my side of things and that he wasn't there at the time so couldn't and wouldn't make any judgements

I talked to mT about the same old therapist and he had no such hesitations