Two pumps are used on CIS / K jet injected cars because they need high fuel pressure ( 100 PSI max I think ) The pumps are in series, first pump supercharges the second.

The smaller fat can is a accumulator, it holds pressure when the engine is off. This reduces vapor lock on a hot engine. The small hose that leads back to the fuel supply hose is the non pressurized side of the diaphragm. On older cars they was open to atmosphere so the results when it failed are predictable.