In the case of this engine, when I put two CC-M modules in parallel, the trailing unit was not happy. After looking at the pictures on the 'scope, I could see the signals were marginal. I have no idea if it's just this particular set or a general issue, but since I've experienced a host of issues with the serial data stream that buffering it solved, I'm just added the boost for the PWM signals.
FWIW, I also use the PWM boost on my Smart Motherboard project, I found that the single DCDR (or CC-M) loaded down the signals enough that they couldn't be recognized by the on-board logic.
The issue with the serial data stream and CC-M problems are the stuff of legends, that's been talked about for years. Even with the most recent CC-M, more than one person has encountered serial data issues.
Whenever you add things in a non-standard configuration, you have to think about the possible issues you may be introducing.
