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Years ago we used to have r/f in only one direction. But as the human race developed, our need for larger shelter increased. Thus occurance of slab fall event become commen for slab with r/f in only one direction.
So peoples tried to know the reasons behind this. As from the study they concluded that, not all large area slabs were failing but only slab with larger area & square c/s otherwise slab with rectangular shape doesn't showed much failing incidents.
Thus they concluded that, in case of square slab the reinforcement in only one direction was unable to take load of slab due to reason that in case of square slab the deflection of slab is dominant in both directions. Thus they came up with a solution : providing reinforcement in both directions.
But providing the two way reinforcement in every slab become costly & uneconomical as not every slab needed that much of reinforcement.
So they have to come up with some formula/ which will tell them where to provide 1 way r/f and where 2 way r/f.
But no one can predict behavior of slab. Thus they failed to come up with any formula so they started to study various slabs behavior.
So after studying various case studies, lots of research they came up with conclusion that if the ration of longer soan length to the shorter span length is greater than two then only we can provide one way reinforcement safely otherwise we have to provide 2 way reinforcement.
So the number two is just a random number fixed after lots of reasearch and practical case studies(also it make calculations easy) That number doesn't have any specific formula or somthing like that it's just a number assumed based experiences & case studies.
Most of the values from IS codes(or your counrty code) are fixed/taken/arrived from the reasearch on various case studies or experience or failure incidents happened. Thus they don't have any specific explanation or formula. Don't think much /waste time about/on them ..