The engines are supplied from the collector cells. Fuel is stored in a collector cell and 3 other fuel cells (a total of 4 cells for Forward tank and 4 for Aft tank).
An ejector valve in each fuel system (or "tank"), "powered" by booster pump fuel pressure, draws fuel from the other 3 fuel cells to keep the Collector cell full. When the collector cell is full, the level control valve closes, thus stopping the supply of fuel from the other 3 cells. This is the only way to stop the flow of fuel into the collector cell as the booster pumps cannot be selected off in flight. As the Collector cell empties, the Level Control Valve opens to allow the Collector cell to be replenished.
This is intended to ensure that the Collector cell will be the last one to run out of fuel.
Hope this helps.
If you need a schematic, PM me and I will try to scan and send the one I have. Please note that I came off the Twin Otter about 15 years ago and have not kept current on any system changes.