We are not talking about about two subscriptions under two different Adobe IDs. As of late October 2017:
Under one Adobe ID (#1) I own a perpetual license to Lightroom 6.
Under another Adobe ID (#2) I have my full CC subscription.
With all of the programs I am fine with the new CC applications, except for Lightroom. I use them, and I pay as I go. If I ever take a year off, or retire, or whatever, not big deal. I got the use of the programs as I paid for them.
Lightroom is different. With Lightroom we are building archives of all of our photography, our life's work. If I stop my subscription at some point in the future, I don't want to be cut off from my photography, from my thousands of hours of archival corrections and ratings.
Lightroom has a different purpose. It's a different animal. So I want to use all of the CC apps except for Lightroom. For Lightroom, I want to continue using Lightroom 6.
So I am legally paying for and have the right to both my CC subscription apps, and to my perpetual licensed Lightroom 6 app.
The problem is that since the Oct 2017 introduction of Lightroom Classic CC and Lightroom CC (the new online version), every time that I sign out of my Adobe ID #1 (Perpetual) and into my Adobe ID #2 (Subscription), Adobe Creative Cloud replaces my Lightroom 6 with the new Lightroom Classic CC.
Not during the Update process in the menu (which has a checkbox for "Leave previous versions in place").
It replaces and deletes Lightroom 6 simple just by signing in. Every single time.