That makes no sense. Even with a single belt, the alternator is getting driven by the exact same contact area as it did when completely stock. IT's roughly a 50% contact area, about the best you can get. And the alternator doesnt present that much load anyway.

Whereas the water pump/fan presents much more load because of the cumbersome fan kicking in, and by removing the airpump belt you reduce contact area from roughly 40% down to roughly 20%. Running the double belt restores the contact area and stops it from slipping.

I think your single belt was just too loose to begin with.

 

I agree. If the belt was loose enough to make the laternator slip, you'd have serious cooling problems, because the alternator pulley would've have far more grip than the water pump pulley liike Kevin said. If the belt wasn't loose, perhaps you shouyld get the alternator checked.