Sorry – I uploaded the wrong PDF. Now corrected.
The PDF mentions a previous job like this, but there was a third guy who did it, years previously, but he vanished as soon as someone told him that it wasn’t legal. He was told by an FSDO inspector that any engine on the same engine TC can be fitted to the aircraft, so, hey, he went and did exactly that 
Fitting a second magneto? The TB20/21 allready has dual magneto, but in a common housing with single shaft drive. What is the motivation or concern here… shaft failure or something else?
The PDF contains the reasoning.
Personally, for the effort, I would fry some bigger fish, like getting 9:1 pistons installed and get a bit more power for the same fuel flow. It is routinely done in the US Exp market. But this is a reasonable mod, especially as it dramatically improves the electronic ignition path.
Are there any hard evidence to support this idea of two individual magnetos beeing more safe than a dual setup?
They probably are safer, plus you get the easier electronic ignition certification (see the PDF, and the D3000 electronic ignition thread) but the difference is marginal. There have been a few failures of the dual mags but the numbers (well, the ones which were posted on forums) are very small.
Peter wrote:
Sorry – I uploaded the wrong PDF. Now corrected.
Thanks! Very interesting indeed.
THY wrote:
What is the motivation or concern here… shaft failure or something else?
The original dual magnetos are in one housing and on a single shaft. So e.g. magnetos can not be overhauled individually, only both of them together. This kind of dual-magneto also precludes any sort of electronic ignition (which usually replaces one magneto but not both). Apparently also the parts situation for those rather specific magnetos is not ideal. And there is the single shaft issue, however I wonder how much of a safety concern that really is.
With two separate magnetos all those issues go away. You can maintain and overhaul the magnetos separately, you can fit electronic ignition and you have a broader choice of magnetos to choose from, Bendix, Slick or as mentioned electronic ignition.
