Thanks Nick. Seems you may be right. I loaded up the 2024 version to use the 24hr historical weather, and tested it a little further. For reference it was at CYWL in the early hours just before sunrise yesterday (though dev mode might allow you to force icing).
This time, I ran the right prop heat. Same result. Both props picked up speed. So a single system for now.
The failures tab has only a single entry for "prop deice" so that is a clue too. Perhaps you ran into that when programming the aircraft. I also tried failing it, for science. To my surprise, the gauge still registered current and the system still worked - props picked up RPM. So that may be another one to check as part of the same investigation.
A somewhat unimportant detail not to lose sleep over, but I can imagine that it'll bug you now you know it's there (sorry)
Thanks,
Martyn