The problem is not with the belt being able to withstand the tension, the problem is that a single belt in a single pulley has only half the surface area to grip than does a double pulley/belt arrangement. To ensure that the single belt does not slip, additional tension is required.
OK, a modern belt construction might be better able to withstand the tension needed to prevent slippage, but what about the rest? What does that addition tension do to the Generator/Fan mounting?
http://www.mapleleafup.net/forums/sh...439#post268439
To avoid overstraining any fragile castings or prematurely causing bearing failure, you want to keep the belt tension as low as possible without allowing slippage. The double belt/pulley arrangement allows a lower tension. Combined with the ball bearing shaft in the double water pumps (compared to a plain bronze bush in a single pulley pump), this system gives a much longer, reliable life.