A household can receive two unrelated water bills from two organisations with no obvious connection to each other, and neither invoice explains what the other one is for. For anyone new to the country it looks like being charged twice for the same thing, which is a reasonable thing to want cleared up.
Different organization entirely, doing a different job. Waterschapsbelasting comes from your regional water board (waterschap) — there are about 21 of them — and it funds flood defense, dike maintenance, regional water quality, and in some areas water-related road upkeep. Your drinking water bill comes from a separate water utility company (Vitens, Dunea, Evides, PWN, and others, depending on where you live), which is purely about supplying and metering the water that comes out of your tap. They don't share billing or a parent organization, which is why you get two unrelated invoices for what feels like the same thing. The water board bill usually includes a couple of components — watersysteemheffing, which funds flood and water management, and, if you're a tenant, zuiveringsheffing for wastewater treatment — often billed via a regional tax office like BsGW rather than the water board itself. It's a genuinely Dutch quirk of having centuries-old water management authorities that predate the modern municipal system.