Whilst I agree serpentine belts are the ultimate for these applications, I would have to say that in 7 years we have had no issues with the twin belt setup on our Yanmar with Leece Neville 175A alternator - and we have an external regulator which means that the alternator works harder than a standard one.
I changed the belts 5 years ago, just because I thought I should, and we had only just noticeable dusting and never broken or worn out a belt.
In my previous experience, belt dusting and short belt life is caused by one or both of two things
1. Pulley misalignment
2, damaged V grooves in either or both pulleys - the groove sides must be absolutely smooth and clean.
and of course, incorrect belt tension.
A recommendation for belts I got from a local belts and transmission supplier, years ago, is to use Dayco / Carlisle raw edged cogged belts. They go around corners better and offer superior grip on the V pulleys than solid belts.
I have been using those belts on this boat and the previous one for nearly 15 years with no issues ever.
An advantage to using the standard setup, on our boat anyway, is that all 3 belts on the front of the engine are the same size.