A sheave (pronounced "shiv" or "shēv") is a wheel with a groove along its edge for holding a belt. When hung between two supports and equipped with a belt, one or more sheaves make up a pulley. The words sheave and pulley are sometimes used interchangeably. In automotive, both sheaves are fixed to their pulley and do not turn independly from each other.
It is important to note that two different diameter (or width) sheaves can make up a pulley (like on your water pump). Y-Blocks with superchargers use this.
If your system uses multi-sheaves on the same pulley and both belts share another multi-sheave pulley, each independent pulley needs to use same-diameter sheaves. In our case, the water pump pulley may have three sheaves but the two that are the same diameter will go to the crank pulley. The third (odd diameter) sheave will be used independently.
I know some damper pulleys (on the crank) are different diameters. The fight starts and heat builds, melting urethane on the damper ring, when both belts are used with different diameter sheaves.
But there is nothing wrong with using muliple belts on same diameter sheaves, even if the water pump diameter is different. Belt speed is determined by the damper pulley.
Simply put, if your sheaves are all different diameters, they cannot be shared with the damper pulley. Only one belt can. - Dave