What is the indication of both ends of a spring dynamometer with the same force? For example, if two forces are 5N, why is the indicator also 5N instead of adding or subtracting (composition / decomposition of forces)?

What is the indication of both ends of a spring dynamometer with the same force? For example, if two forces are 5N, why is the indicator also 5N instead of adding or subtracting (composition / decomposition of forces)?

The action of forces is mutual. One spring dynamometer gives a force of 5N to another spring dynamometer, and the second spring dynamometer also gives a force of 5N to the first spring dynamometer