First off this would only work if you were only running just your waterpump and alternator off the crank. But, instead of running a dual alt pully with 2 belts, could your convert it to a serp. belt?? this would eliminate slipping, but you would have to find the right pullies somwhere. This might have already been done, or might not be possible, its just a thought.
Josh

 

It's possible