Looking at the photo I think it is a chain driven dynamo rather than a second magneto.
When I bought my 2 Speeder as a restoration project it had a double sprocket on the magneto for a chain driven dynamo. I assume this was because the bike was not fitted with a magdyno when new. It was more economic for the owner to add a dynamo rather than buy a magdyno when he wanted to fit lights.
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