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Apollo lunar spacesuits had four hose connectors for air:
The EV PGA, however, is provided with two sets: two [blue] inlet connectors interconnected by a plenum chamber and two [red] outlet connectors interconnected the same way.
Apollo Experience Report: Development of the Extravehicular Mobility Unit
Why are there two sets of connectors?
Related:
From History of the Apollo Space Suit by the International Latex Corporation (ILC). PDF
When setting up the suits in preparation for the extravehicular walk on the lunar surface, the astronauts attached oxygen hoses from the Lunar Module (both inlet and outlet) while at the same time attaching to the inlet and outlet hoses of the portable backpack. Once the pressure in the Lunar Module was reduced to the hard vacuum of the moon and the suits were pressurized at 3.8 lb/in² (26 kPa), the backpack would become the sole source for the suit pressure and the astronauts would disconnect from the Lunar Module and descend the ladder to the surface. The suit inlet and outlet fitting for the Lunar Module interface both sealed closed once the hoses were removed. They would reverse the process upon reentry to the Lunar Module.
The double set of gas connectors with pairs of inlets and outlets was used to connect both the LM life support system as well as the backpack of the extravehicular suit. An uninterrupted transfer between both life support systems was enabled by the pairs of connectors.
The intravehicular suit version worn by the CM pilot had only a single set of gas connectors. It was only connected to the suit circuit of the CM and never to a PLSS backpack.
From the APOLLO SPACE SUIT INTERFACE SPECIFICATION, 18 April 1962 PDF.
It was planned to use one PLSS for two astronauts in emergency. The hoses of the other astronaut with the failed PLSS would be connected to the second pair of the gas connectors at the suit of the first astronaut with the functional PLSS.
