From the context, it seems the OP has wheels and brakes that he's transferring to the other (non-disc) frame. Looking at the photo of the Brake therapy adapter, it seems that it will fill the bill nicely, with the only real drawback being more work removing the rear wheel.
The adapter is well thought out, and nicely transfers braking torque up to the Canti boss where the frame is designed to take the load. I might mention that the full rather than half load is going to a single stay, doubling the stress, but I doubt the stays were engineered that close anyway.
The OP should see if he can get user reviews of the adapter, and if it's well made and has good reviews he's found a nice solution to his problem
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